fix(tabletop): tilt cards uniformly about the y axis

Apply the same tilt to every stacked part and rotate it about its local
Y axis instead of Z. Default tilt to 1 degree when a stacking strategy
is present, and arc the poker deck along the surface's bottom edge.

docs/bgm-format.md

@@ -371,10 +371,9 @@ layout:
 - `align` — `start`, `end`, or `center` of the curve.
 - `steps` — the maximum number of parts per curve length unit. Defaults to
   `1`. See the positioning process below.
-- `tilt` — rotation in degrees per shown part about the card's local Y (long)
-  axis. Each part tilts `tilt` more than the previous, fanning the stack so
-  its edges stay visible. It applies even without a `curve`, so a bare `tilt`
-  fans a straight pile.
+- `tilt` — rotation in degrees applied to every shown part about the card's
+  local Y (long) axis. It applies even without a `curve`, so a bare `tilt`
+  rotates a straight pile.
 - `zStart` / `zEnd` — the height (surface-normal) in mm at the start and end
   of the `curve`. The stack ramps linearly between them across its span,
   lifting it in 3D. Requires a `curve`.
@@ -389,8 +388,8 @@ layout:
    each `step length` apart.
 4. **Lift each part.** The part's height is `zStart + (zEnd − zStart) × u`,
    where `u` is its normalized position along the `curve`.
-5. **Tilt each part.** Each part is rotated `tilt × # of parts before it`
-   about its local Y (long) axis.
+5. **Tilt each part.** Every part is rotated `tilt` about its local Y (long)
+   axis.
 
 ### Edge cases
 

docs/bgm-tabletop-plan.md

@@ -123,7 +123,7 @@ interface GameState {
   length from curve length / `max(steps, count-1)`, alignment (`start`/`end`/
   `center`), and `limit` (`0` all, `n` first n, `-n` last n).
 - `z` ramps linearly from `zStart` to `zEnd` across the curve's span; `tilt`
-  fans each shown part about its local Y (long) axis.
+  rotates each shown part about its local Y (long) axis.
 - Curve length from an SVG path string (small helper; no new dep).
 
 ### 8. Public API (`index.ts`) ✅

docs/bgm-tabletop.md

@@ -40,7 +40,7 @@ the render map is per enabled surface: a piece may appear on more than one enabl
 
 ## 4. stacking
 
-the format's stacking strategy (`curve` / `limit` / `align` / `steps` / `tilt` / `zStart` / `zEnd`, see `bgm-format.md` §4) is implemented as a hook, e.g. `useStacking(route.stacking, index, stackSize)`, returning the offset/rotation to apply to a piece: `{ x, y, rotation, z, tilt }`. `x`/`y`/`rotation` come from the `curve`; `z` is the surface-normal height ramped from `zStart` to `zEnd`; `tilt` is the per-part fan about the card's local Y (long) axis. `PartPlacement` consumes it.
+the format's stacking strategy (`curve` / `limit` / `align` / `steps` / `tilt` / `zStart` / `zEnd`, see `bgm-format.md` §4) is implemented as a hook, e.g. `useStacking(route.stacking, index, stackSize)`, returning the offset/rotation to apply to a piece: `{ x, y, rotation, z, tilt }`. `x`/`y`/`rotation` come from the `curve`; `z` is the surface-normal height ramped from `zStart` to `zEnd`; `tilt` is the rotation about the card's local Y (long) axis, applied to every part. `PartPlacement` consumes it.
 
 ## 5. usage
 

games/poker/poker.md

@@ -103,12 +103,11 @@ layout:
     y: 0
     rotation: 0
     stacking:
-      curve: M 0 0 C 20 -20 40 -20 60 0
-      limit: 0
       align: center
-      tilt: 0.015
       zStart: 0
-      zEnd: 10
+      #zEnd: 100
+      tilt: 1
+      curve: M -50 -200 C 50 -150 450 -150 550 -200
   - route: /community/:slot
     candidates:
       $variants: ./community.csv

packages/tabletop/src/placement.tsx

@@ -21,7 +21,7 @@ export function PartPlacement({ pkg, placement }: { pkg: Package; placement: Pla
 
   // Route anchors and stacking offsets are in mm; convert to world units so
   // parts land on the (world-scaled) surface. `z` raises the part along the
-  // surface normal (world +Y); `tilt` fans it about its local Y (long) axis.
+  // surface normal (world +Y); `tilt` rotates it about its local Y (long) axis.
   // Angles are authored in degrees; three.js expects radians.
   const anchorX = ((candidate?.x ?? route.x ?? 0) + x) * MM_TO_WORLD;
   const anchorY = ((candidate?.y ?? route.y ?? 0) + y) * MM_TO_WORLD;
@@ -31,7 +31,7 @@ export function PartPlacement({ pkg, placement }: { pkg: Package; placement: Pla
   return (
     <group position={[anchorX, anchorZ, anchorY]} rotation={[0, anchorRotation, 0]}>
       {/* The part mesh extrudes along +Z; lay it flat so its face points up. */}
-      <group rotation={[-Math.PI / 2, 0, tilt * DEG_TO_RAD]}>
+      <group rotation={[-Math.PI / 2, tilt * DEG_TO_RAD, 0]}>
         <PartView part={part} baseUrl={part.baseUrl} />
       </group>
     </group>

packages/tabletop/src/stacking.test.ts

@@ -46,7 +46,8 @@ describe('pointAt', () => {
 describe('stackingOffset', () => {
   it('returns no offset without a curve', () => {
     expect(stackingOffset(undefined, 0, 3)).toBe(NO_OFFSET);
-    expect(stackingOffset({ limit: 5 }, 0, 3)).toBe(NO_OFFSET);
+    // A stacking strategy defaults to a 1° tilt, so it's not the identity.
+    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', () => {
@@ -86,12 +87,12 @@ describe('stackingOffset', () => {
     expect(offset.x).toBeCloseTo(25);
   });
 
-  it('tilts each part 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: 0.2 });
+    expect(offset).toEqual({ x: 0, y: 0, rotation: 0, z: 0, tilt: 0.1 });
   });
 
-  it('tilts parts along the curve', () => {
+  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);
@@ -113,7 +114,7 @@ describe('stackingOffset', () => {
     expect(last.z).toBeCloseTo(40);
   });
 
-  it('returns no offset without a curve, tilt, or z ramp', () => {
-    expect(stackingOffset({ limit: 5 }, 0, 3)).toBe(NO_OFFSET);
+  it('returns no offset without a stacking strategy', () => {
+    expect(stackingOffset(undefined, 0, 3)).toBe(NO_OFFSET);
   });
 });

packages/tabletop/src/stacking.ts

@@ -43,9 +43,10 @@ export function stackingOffset(
   const shownIndex = shown.indexOf(index);
   if (shownIndex < 0) return NO_OFFSET;
 
-  // `tilt` fans each shown part about its local Y (long) axis, so the stack's
-  // edges stay visible. It applies even without a curve.
-  const tilt = (stacking?.tilt ?? 0) * shownIndex;
+  // `tilt` rotates each shown part about its local Y (long) axis by the same
+  // amount. It applies even without a curve. Defaults to 1° when a stacking
+  // strategy is present but doesn't specify a tilt.
+  const tilt = stacking?.tilt ?? (stacking ? 1 : 0);
 
   // The horizontal position along the curve (or a straight pile when there's
   // no curve), plus the normalized progress used to ramp the z height.