diff --git a/packages/tabletop/src/interactions.test.ts b/packages/tabletop/src/interactions.test.ts
index 0c8135a..99d70fa 100644
--- a/packages/tabletop/src/interactions.test.ts
+++ b/packages/tabletop/src/interactions.test.ts
@@ -1,28 +1,29 @@
-import { describe, expect, it } from 'vitest';
-import type { Package, Setup } from '@tts/bgm';
+import { beforeEach, describe, expect, it } from "vitest";
+import type { Package, Setup } from "@tts/bgm";
 import {
   interactionsFor,
   dropPaths,
   pickPath,
   partFacings,
   nextFacing,
-} from './interactions.js';
+  useInteractionStore,
+} from "./interactions.js";
 
 const pkg: Package = {
-  meta: { id: 'harbor' },
+  meta: { id: "harbor" },
   parts: new Map([
-    ['card#a', { type: 'card', id: 'a' }],
-    ['card#b', { type: 'card', id: 'b', facing: ['face', 'back'] }],
+    ["card#a", { type: "card", id: "a" }],
+    ["card#b", { type: "card", id: "b", facing: ["face", "back"] }],
   ]),
   surfaces: new Map([
     [
-      'board#harbor',
+      "board#harbor",
       {
-        type: 'board',
-        id: 'harbor',
+        type: "board",
+        id: "harbor",
         layout: [
-          { route: '/deck', x: 0, y: 0, rotation: 0 },
-          { route: '/discard', x: 40, y: 0, rotation: 0 },
+          { route: "/deck", x: 0, y: 0, rotation: 0 },
+          { route: "/discard", x: 40, y: 0, rotation: 0 },
         ],
       },
     ],
@@ -32,77 +33,156 @@ const pkg: Package = {
 };
 
 const setup: Setup = {
-  type: 'game',
-  id: 'main',
+  type: "game",
+  id: "main",
   setup: [],
   interactions: [
-    { dialog: 'prompt#insert', on: ['/deck'] },
-    { dialog: 'prompt#shuffle' },
+    { dialog: "prompt#insert", on: ["/deck"] },
+    { dialog: "prompt#shuffle" },
   ],
 };
 
-describe('interactionsFor', () => {
-  it('returns the interactions whose `on` matches the path, plus any without `on`', () => {
-    expect(interactionsFor(setup, '/deck').map((i) => i.dialog)).toEqual([
-      'prompt#insert',
-      'prompt#shuffle',
+describe("interactionsFor", () => {
+  it("returns the interactions whose `on` matches the path, plus any without `on`", () => {
+    expect(interactionsFor(setup, "/deck").map((i) => i.dialog)).toEqual([
+      "prompt#insert",
+      "prompt#shuffle",
     ]);
   });
 
-  it('includes interactions without `on` for any path', () => {
-    expect(interactionsFor(setup, '/discard').map((i) => i.dialog)).toEqual([
-      'prompt#shuffle',
+  it("includes interactions without `on` for any path", () => {
+    expect(interactionsFor(setup, "/discard").map((i) => i.dialog)).toEqual([
+      "prompt#shuffle",
     ]);
   });
 
-  it('returns [] when the setup declares no interactions', () => {
+  it("returns [] when the setup declares no interactions", () => {
     expect(
-      interactionsFor({ type: 'game', id: 'main', setup: [] }, '/deck'),
+      interactionsFor({ type: "game", id: "main", setup: [] }, "/deck"),
     ).toEqual([]);
   });
 });
 
-describe('dropPaths', () => {
-  it('uses the declared `on` paths when interactions exist', () => {
-    expect([...dropPaths(setup, pkg)]).toEqual(['/deck']);
+describe("dropPaths", () => {
+  it("uses the declared `on` paths when interactions exist", () => {
+    expect([...dropPaths(setup, pkg)]).toEqual(["/deck"]);
   });
 
-  it('falls back to every literal routed path when no interactions are declared', () => {
-    const bare: Setup = { type: 'game', id: 'main', setup: [] };
-    expect([...dropPaths(bare, pkg)].sort()).toEqual(['/deck', '/discard']);
+  it("falls back to every literal routed path when no interactions are declared", () => {
+    const bare: Setup = { type: "game", id: "main", setup: [] };
+    expect([...dropPaths(bare, pkg)].sort()).toEqual(["/deck", "/discard"]);
+  });
+
+  it("skips :param routes in the fallback, since they need a candidate", () => {
+    const paramPkg: Package = {
+      ...pkg,
+      surfaces: new Map([
+        [
+          "board#harbor",
+          {
+            type: "board",
+            id: "harbor",
+            layout: [
+              { route: "/dock/:seat", x: 0, y: 0, rotation: 0 },
+              { route: "/deck", x: 40, y: 0, rotation: 0 },
+            ],
+          },
+        ],
+      ]),
+    };
+    const bare: Setup = { type: "game", id: "main", setup: [] };
+    expect([...dropPaths(bare, paramPkg)]).toEqual(["/deck"]);
   });
 });
 
-describe('pickPath', () => {
-  it('returns the nearest literal route within the threshold', () => {
-    expect(pickPath(pkg, 'board#harbor', 41, 1, 40)).toBe('/discard');
+describe("pickPath", () => {
+  it("returns the nearest literal route within the threshold", () => {
+    expect(pickPath(pkg, "board#harbor", 41, 1, 40)).toBe("/discard");
   });
 
-  it('returns null when nothing is within the threshold', () => {
-    expect(pickPath(pkg, 'board#harbor', 100, 100, 40)).toBeNull();
+  it("returns null when nothing is within the threshold", () => {
+    expect(pickPath(pkg, "board#harbor", 100, 100, 40)).toBeNull();
   });
 
-  it('returns null for an unknown surface', () => {
-    expect(pickPath(pkg, 'board#nope', 0, 0, 40)).toBeNull();
+  it("returns null for an unknown surface", () => {
+    expect(pickPath(pkg, "board#nope", 0, 0, 40)).toBeNull();
+  });
+
+  it("skips :param routes, which have no fixed anchor", () => {
+    const paramPkg: Package = {
+      ...pkg,
+      surfaces: new Map([
+        [
+          "board#harbor",
+          {
+            type: "board",
+            id: "harbor",
+            layout: [
+              { route: "/dock/:seat", x: 0, y: 0, rotation: 0 },
+              { route: "/deck", x: 40, y: 0, rotation: 0 },
+            ],
+          },
+        ],
+      ]),
+    };
+    expect(pickPath(paramPkg, "board#harbor", 100, 100, 40)).toBeNull();
+    expect(pickPath(paramPkg, "board#harbor", 41, 1, 40)).toBe("/deck");
   });
 });
 
-describe('partFacings / nextFacing', () => {
-  it('defaults to the full set when the part declares none', () => {
-    expect(partFacings(pkg.parts.get('card#a')!)).toEqual([
-      'face',
-      'back',
-      'standing',
+describe("useInteractionStore", () => {
+  beforeEach(() => {
+    useInteractionStore.setState({ held: null, dialogs: [] });
+  });
+
+  it("holds and drops a part", () => {
+    useInteractionStore
+      .getState()
+      .hold({ id: "harbor:card#a", origin: "/deck" });
+    expect(useInteractionStore.getState().held).toEqual({
+      id: "harbor:card#a",
+      origin: "/deck",
+    });
+    useInteractionStore.getState().drop();
+    expect(useInteractionStore.getState().held).toBeNull();
+  });
+
+  it("pushes and pops the dialog stack", () => {
+    const { pushDialog, popDialog } = useInteractionStore.getState();
+    pushDialog({ id: "prompt#insert", path: "/deck" });
+    pushDialog({ id: "prompt#shuffle", path: "/deck" });
+    expect(useInteractionStore.getState().dialogs).toEqual([
+      { id: "prompt#insert", path: "/deck" },
+      { id: "prompt#shuffle", path: "/deck" },
+    ]);
+    popDialog();
+    expect(useInteractionStore.getState().dialogs).toEqual([
+      { id: "prompt#insert", path: "/deck" },
+    ]);
+  });
+});
+
+describe("partFacings / nextFacing", () => {
+  it("defaults to the full set when the part declares none", () => {
+    expect(partFacings(pkg.parts.get("card#a")!)).toEqual([
+      "face",
+      "back",
+      "standing",
     ]);
   });
 
-  it('uses the declared affordance', () => {
-    expect(partFacings(pkg.parts.get('card#b')!)).toEqual(['face', 'back']);
+  it("uses the declared affordance", () => {
+    expect(partFacings(pkg.parts.get("card#b")!)).toEqual(["face", "back"]);
   });
 
-  it('cycles forward through the affordance', () => {
-    const part = pkg.parts.get('card#b')!;
-    expect(nextFacing(part, 'face')).toBe('back');
-    expect(nextFacing(part, 'back')).toBe('face');
+  it("cycles forward through the affordance", () => {
+    const part = pkg.parts.get("card#b")!;
+    expect(nextFacing(part, "face")).toBe("back");
+    expect(nextFacing(part, "back")).toBe("face");
+  });
+
+  it("wraps to the first facing after the last", () => {
+    const part = pkg.parts.get("card#b")!;
+    expect(nextFacing(part, "back")).toBe("face");
   });
 });
diff --git a/packages/tabletop/src/stacking.test.ts b/packages/tabletop/src/stacking.test.ts
index f9e965e..fdc87d3 100644
--- a/packages/tabletop/src/stacking.test.ts
+++ b/packages/tabletop/src/stacking.test.ts
@@ -1,131 +1,238 @@
-import { describe, expect, it } from 'vitest';
-import { stackingOffset, parsePath, pointAt, NO_OFFSET } from './stacking.js';
+import { describe, expect, it } from "vitest";
+import { stackingOffset, parsePath, pointAt, NO_OFFSET } from "./stacking.js";
 
-describe('parsePath', () => {
-  it('measures a straight line', () => {
-    const path = parsePath('M 0 0 L 10 0');
+describe("parsePath", () => {
+  it("measures a straight line", () => {
+    const path = parsePath("M 0 0 L 10 0");
     expect(path.length).toBeCloseTo(10);
   });
 
-  it('measures a cubic curve', () => {
-    const path = parsePath('M 0 0 C 20 -20 40 -20 60 0');
+  it("measures a cubic curve", () => {
+    const path = parsePath("M 0 0 C 20 -20 40 -20 60 0");
     // Longer than the chord (60) but finite.
     expect(path.length).toBeGreaterThan(60);
     expect(path.length).toBeLessThan(80);
   });
 
-  it('handles relative commands', () => {
-    const path = parsePath('m 0 0 l 10 0 l 0 10');
+  it("handles relative commands", () => {
+    const path = parsePath("m 0 0 l 10 0 l 0 10");
     expect(path.length).toBeCloseTo(20);
   });
 
-  it('supports h/v/z', () => {
-    const path = parsePath('M 0 0 H 10 V 10 Z');
+  it("supports h/v/z", () => {
+    const path = parsePath("M 0 0 H 10 V 10 Z");
     // 10 right + 10 down + the diagonal back to the start (closes the triangle).
     expect(path.length).toBeCloseTo(10 + 10 + Math.sqrt(200));
   });
+
+  it("supports quadratic curves (Q)", () => {
+    const path = parsePath("M 0 0 Q 50 50 100 0");
+    // Longer than the chord (100) but finite.
+    expect(path.length).toBeGreaterThan(100);
+    expect(path.length).toBeLessThan(120);
+    // The curve passes through the midpoint of the control point.
+    const mid = pointAt(path, path.length / 2);
+    expect(mid.y).toBeGreaterThan(0);
+  });
+
+  it("supports smooth quadratic continuation (T)", () => {
+    const path = parsePath("M 0 0 Q 50 50 100 0 T 200 0");
+    // Two quadratic segments; the second reflects the first's control point.
+    expect(path.length).toBeGreaterThan(200);
+  });
+
+  it("supports smooth cubic continuation (S)", () => {
+    const path = parsePath("M 0 0 C 25 50 75 50 100 0 S 175 -50 200 0");
+    expect(path.length).toBeGreaterThan(200);
+  });
+
+  it("supports elliptical arcs (A)", () => {
+    // A semicircle of radius 50: length ≈ π * 50.
+    const path = parsePath("M 0 0 A 50 50 0 0 1 100 0");
+    expect(path.length).toBeCloseTo(Math.PI * 50, 0);
+  });
+
+  it("supports relative variants of each command", () => {
+    const path = parsePath("m 0 0 q 50 50 100 0 t 100 0");
+    expect(path.length).toBeGreaterThan(200);
+  });
+
+  it("throws on an unsupported command", () => {
+    expect(() => parsePath("M 0 0 R 10 10")).toThrow(
+      /Unsupported SVG path command/,
+    );
+  });
 });
 
-describe('pointAt', () => {
-  it('returns the start at distance 0 and end at full length', () => {
-    const path = parsePath('M 0 0 L 10 0');
+describe("pointAt", () => {
+  it("returns the start at distance 0 and end at full length", () => {
+    const path = parsePath("M 0 0 L 10 0");
     expect(pointAt(path, 0)).toMatchObject({ x: 0, y: 0 });
     const end = pointAt(path, path.length);
     expect(end.x).toBeCloseTo(10);
     expect(end.y).toBeCloseTo(0);
   });
 
-  it('interpolates along the path', () => {
-    const path = parsePath('M 0 0 L 10 0');
+  it("interpolates along the path", () => {
+    const path = parsePath("M 0 0 L 10 0");
     const mid = pointAt(path, 5);
     expect(mid.x).toBeCloseTo(5);
     expect(mid.angle).toBeCloseTo(0);
   });
 
-  it('returns the tangent angle in degrees', () => {
-    const path = parsePath('M 0 0 L 10 10');
+  it("returns the tangent angle in degrees", () => {
+    const path = parsePath("M 0 0 L 10 10");
     expect(pointAt(path, 5).angle).toBeCloseTo(45);
-    const down = parsePath('M 0 0 L 0 10');
+    const down = parsePath("M 0 0 L 0 10");
     expect(pointAt(down, 5).angle).toBeCloseTo(90);
   });
 
-  it('returns the tangent angle at the start of the path', () => {
-    const path = parsePath('M 0 0 L 10 10');
+  it("returns the tangent angle at the start of the path", () => {
+    const path = parsePath("M 0 0 L 10 10");
     expect(pointAt(path, 0).angle).toBeCloseTo(45);
   });
+
+  it("clamps distance to the path length", () => {
+    const path = parsePath("M 0 0 L 10 0");
+    expect(pointAt(path, 999).x).toBeCloseTo(10);
+    expect(pointAt(path, -5).x).toBeCloseTo(0);
+  });
+
+  it("returns the origin for an empty path", () => {
+    expect(pointAt({ points: [], length: 0 }, 5)).toEqual({
+      x: 0,
+      y: 0,
+      angle: 0,
+    });
+  });
+
+  it("returns the single point for a degenerate path", () => {
+    const path = parsePath("M 5 5");
+    expect(pointAt(path, 0)).toEqual({ x: 5, y: 5, angle: 0 });
+  });
 });
 
-describe('stackingOffset', () => {
-  it('defaults to a 1° tilt without a curve', () => {
-    expect(stackingOffset(undefined, 0, 3)).toEqual({ x: 0, y: 0, rotation: 0, z: 0, tilt: 1 });
-    expect(stackingOffset({ limit: 5 }, 0, 3)).toEqual({ x: 0, y: 0, rotation: 0, z: 0, tilt: 1 });
+describe("stackingOffset", () => {
+  it("defaults to a 1° tilt without a curve", () => {
+    expect(stackingOffset(undefined, 0, 3)).toEqual({
+      x: 0,
+      y: 0,
+      rotation: 0,
+      z: 0,
+      tilt: 1,
+    });
+    expect(stackingOffset({ limit: 5 }, 0, 3)).toEqual({
+      x: 0,
+      y: 0,
+      rotation: 0,
+      z: 0,
+      tilt: 1,
+    });
   });
 
-  it('spreads parts evenly along a straight curve', () => {
-    const offset = stackingOffset({ curve: 'M 0 0 L 100 0' }, 1, 3);
+  it("spreads parts evenly along a straight curve", () => {
+    const offset = stackingOffset({ curve: "M 0 0 L 100 0" }, 1, 3);
     // step = length / max(steps=1, 2) = 50; part 1 at 50.
     expect(offset.x).toBeCloseTo(50);
     expect(offset.y).toBeCloseTo(0);
   });
 
-  it('aligns to center', () => {
-    const offset = stackingOffset({ curve: 'M 0 0 L 100 0', align: 'center' }, 0, 3);
+  it("aligns to center", () => {
+    const offset = stackingOffset(
+      { curve: "M 0 0 L 100 0", align: "center" },
+      0,
+      3,
+    );
     // span = 50 * 2 = 100; centered start = (100 - 100)/2 = 0.
     expect(offset.x).toBeCloseTo(0);
   });
 
-  it('aligns to end', () => {
-    const offset = stackingOffset({ curve: 'M 0 0 L 100 0', align: 'end' }, 2, 3);
+  it("aligns to end", () => {
+    const offset = stackingOffset(
+      { curve: "M 0 0 L 100 0", align: "end" },
+      2,
+      3,
+    );
     // start = 100 - 100 = 0; part 2 at 100.
     expect(offset.x).toBeCloseTo(100);
   });
 
-  it('respects a positive limit (first n)', () => {
-    const offset = stackingOffset({ curve: 'M 0 0 L 100 0', limit: 2 }, 2, 4);
+  it("respects a positive limit (first n)", () => {
+    const offset = stackingOffset({ curve: "M 0 0 L 100 0", limit: 2 }, 2, 4);
     // Part 2 is beyond the first 2 shown -> not placed.
     expect(offset).toBe(NO_OFFSET);
   });
 
-  it('respects a negative limit (last n)', () => {
+  it("respects a negative limit (last n)", () => {
     // Last 2 of 4 are indices 2,3. Part 2 is the first shown.
-    const offset = stackingOffset({ curve: 'M 0 0 L 100 0', limit: -2 }, 2, 4);
+    const offset = stackingOffset({ curve: "M 0 0 L 100 0", limit: -2 }, 2, 4);
     expect(offset.x).toBeCloseTo(0);
   });
 
-  it('uses steps to densify the curve', () => {
+  it("uses steps to densify the curve", () => {
     // steps=4, 3 parts -> step = 100 / max(4, 2) = 25.
-    const offset = stackingOffset({ curve: 'M 0 0 L 100 0', steps: 4 }, 1, 3);
+    const offset = stackingOffset({ curve: "M 0 0 L 100 0", steps: 4 }, 1, 3);
     expect(offset.x).toBeCloseTo(25);
   });
 
-  it('tilts every part the same amount without a curve', () => {
+  it("tilts every part the same amount without a curve", () => {
     const offset = stackingOffset({ tilt: 0.1 }, 2, 3);
     expect(offset).toEqual({ x: 0, y: 0, rotation: 0, z: 0, tilt: 0.1 });
   });
 
-  it('tilts every part the same amount along the curve', () => {
-    const offset = stackingOffset({ curve: 'M 0 0 L 100 0', tilt: 0.1 }, 1, 3);
+  it("tilts every part the same amount along the curve", () => {
+    const offset = stackingOffset({ curve: "M 0 0 L 100 0", tilt: 0.1 }, 1, 3);
     expect(offset.x).toBeCloseTo(50);
     expect(offset.tilt).toBeCloseTo(0.1);
   });
 
-  it('tilts only the shown parts', () => {
+  it("tilts only the shown parts", () => {
     // limit 2 shows indices 0,1; index 2 is dropped.
     expect(stackingOffset({ tilt: 0.1, limit: 2 }, 2, 4)).toBe(NO_OFFSET);
     expect(stackingOffset({ tilt: 0.1, limit: 2 }, 1, 4).tilt).toBeCloseTo(0.1);
   });
 
-  it('ramps z from zStart to zEnd across the curve', () => {
+  it("ramps z from zStart to zEnd across the curve", () => {
     // 3 parts on a 100-long curve: u = 0, 0.5, 1. z ramps 0 -> 40.
-    const first = stackingOffset({ curve: 'M 0 0 L 100 0', zStart: 0, zEnd: 40 }, 0, 3);
-    const mid = stackingOffset({ curve: 'M 0 0 L 100 0', zStart: 0, zEnd: 40 }, 1, 3);
-    const last = stackingOffset({ curve: 'M 0 0 L 100 0', zStart: 0, zEnd: 40 }, 2, 3);
+    const first = stackingOffset(
+      { curve: "M 0 0 L 100 0", zStart: 0, zEnd: 40 },
+      0,
+      3,
+    );
+    const mid = stackingOffset(
+      { curve: "M 0 0 L 100 0", zStart: 0, zEnd: 40 },
+      1,
+      3,
+    );
+    const last = stackingOffset(
+      { curve: "M 0 0 L 100 0", zStart: 0, zEnd: 40 },
+      2,
+      3,
+    );
     expect(first.z).toBeCloseTo(0);
     expect(mid.z).toBeCloseTo(20);
     expect(last.z).toBeCloseTo(40);
   });
 
-  it('returns no offset for an empty stack', () => {
+  it("returns no offset for an empty stack", () => {
     expect(stackingOffset(undefined, 0, 0)).toBe(NO_OFFSET);
   });
-});
\ No newline at end of file
+
+  it("applies only the default tilt when a curve has zero length", () => {
+    // A degenerate curve (a single point) has length 0, so no horizontal
+    // offset applies, but the default 1° tilt still does.
+    expect(stackingOffset({ curve: "M 5 5" }, 0, 3)).toEqual({
+      x: 0,
+      y: 0,
+      rotation: 0,
+      z: 0,
+      tilt: 1,
+    });
+  });
+
+  it("places a single part at the start of the curve", () => {
+    const offset = stackingOffset({ curve: "M 0 0 L 100 0" }, 0, 1);
+    // span = max(steps=1, 0) = 1; step = 100; part 0 at 0.
+    expect(offset.x).toBeCloseTo(0);
+  });
+});
diff --git a/packages/tabletop/src/state.test.ts b/packages/tabletop/src/state.test.ts
index 39f93b9..010599b 100644
--- a/packages/tabletop/src/state.test.ts
+++ b/packages/tabletop/src/state.test.ts
@@ -1,4 +1,4 @@
-import { describe, expect, it } from "vitest";
+import { beforeEach, describe, expect, it } from "vitest";
 import type { Facing, Package, Surface } from "@tts/bgm";
 import {
   matchRoute,
@@ -6,6 +6,7 @@ import {
   computeSurfacePlacements,
   computeRenderState,
   placementKey,
+  useTabletopStore,
 } from "./state.js";
 
 function makeSurface(overrides: Partial<Surface> = {}): Surface {
@@ -212,6 +213,111 @@ describe("computeRenderState", () => {
   });
 });
 
+describe("useTabletopStore", () => {
+  beforeEach(() => {
+    useTabletopStore.setState({ surfaces: {}, parts: {} });
+  });
+
+  it("seeds surfaces and parts", () => {
+    useTabletopStore
+      .getState()
+      .seed({ surfaces: { "board#harbor": true }, parts: {} });
+    expect(useTabletopStore.getState().surfaces).toEqual({
+      "board#harbor": true,
+    });
+  });
+
+  it("enables and disables a surface", () => {
+    useTabletopStore.getState().enableSurface("board#harbor");
+    expect(useTabletopStore.getState().surfaces["board#harbor"]).toBe(true);
+    useTabletopStore.getState().disableSurface("board#harbor");
+    expect(useTabletopStore.getState().surfaces["board#harbor"]).toBe(false);
+  });
+
+  it("setPart patches an existing part and ignores an unknown id", () => {
+    useTabletopStore.getState().setParts({
+      "harbor:card#a": { path: "/deck", index: 0, facing: "face" },
+    });
+    useTabletopStore.getState().setPart("harbor:card#a", { facing: "back" });
+    expect(useTabletopStore.getState().parts["harbor:card#a"]).toEqual({
+      path: "/deck",
+      index: 0,
+      facing: "back",
+    });
+    useTabletopStore.getState().setPart("harbor:card#nope", { facing: "back" });
+    expect(
+      useTabletopStore.getState().parts["harbor:card#nope"],
+    ).toBeUndefined();
+  });
+
+  it("movePart reindexes source and destination, inserting at index", () => {
+    useTabletopStore.getState().setParts({
+      "harbor:card#a": { path: "/deck", index: 0, facing: "face" },
+      "harbor:card#b": { path: "/deck", index: 1, facing: "face" },
+      "harbor:card#c": { path: "/deck", index: 2, facing: "face" },
+    });
+    // Move the top card to the bottom (index 0).
+    useTabletopStore.getState().movePart("harbor:card#c", "/deck", 0);
+    const parts = useTabletopStore.getState().parts;
+    expect(parts["harbor:card#c"]).toEqual({
+      path: "/deck",
+      index: 0,
+      facing: "face",
+    });
+    expect(parts["harbor:card#a"]).toEqual({
+      path: "/deck",
+      index: 1,
+      facing: "face",
+    });
+    expect(parts["harbor:card#b"]).toEqual({
+      path: "/deck",
+      index: 2,
+      facing: "face",
+    });
+  });
+
+  it("movePart moves between paths, closing the source gap", () => {
+    useTabletopStore.getState().setParts({
+      "harbor:card#a": { path: "/deck", index: 0, facing: "face" },
+      "harbor:card#b": { path: "/deck", index: 1, facing: "face" },
+      "harbor:card#c": { path: "/discard", index: 0, facing: "face" },
+    });
+    useTabletopStore.getState().movePart("harbor:card#a", "/discard", 0);
+    const parts = useTabletopStore.getState().parts;
+    expect(parts["harbor:card#a"]).toEqual({
+      path: "/discard",
+      index: 0,
+      facing: "face",
+    });
+    expect(parts["harbor:card#c"]).toEqual({
+      path: "/discard",
+      index: 1,
+      facing: "face",
+    });
+    expect(parts["harbor:card#b"]).toEqual({
+      path: "/deck",
+      index: 0,
+      facing: "face",
+    });
+  });
+
+  it("movePart clamps index and ignores an unknown id", () => {
+    useTabletopStore.getState().setParts({
+      "harbor:card#a": { path: "/deck", index: 0, facing: "face" },
+    });
+    useTabletopStore.getState().movePart("harbor:card#a", "/deck", 99);
+    expect(useTabletopStore.getState().parts["harbor:card#a"]).toEqual({
+      path: "/deck",
+      index: 0,
+      facing: "face",
+    });
+    useTabletopStore.getState().movePart("harbor:card#nope", "/deck", 0);
+    expect(
+      useTabletopStore.getState().parts["harbor:card#nope"],
+    ).toBeUndefined();
+  });
+});
+
 describe("placementKey", () => {
   it("is unique per surface and piece", () => {
     const a = { surface: "board#harbor", piece: "harbor:card#a" } as never;