fix(tabletop): default tilt to 1 degree for all parts

Apply the default 1 degree tilt even when a route has no stacking
strategy, so every placed part is tilted consistently.

docs/bgm-format.md

@@ -373,7 +373,7 @@ layout:
   `1`. See the positioning process below.
 - `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.
+  rotates a straight pile. Defaults to `1` when not specified.
 - `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`.

packages/tabletop/src/stacking.test.ts

@@ -44,9 +44,8 @@ describe('pointAt', () => {
 });
 
 describe('stackingOffset', () => {
-  it('returns no offset without a curve', () => {
-    expect(stackingOffset(undefined, 0, 3)).toBe(NO_OFFSET);
-    // A stacking strategy defaults to a 1° tilt, so it's not the identity.
+  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 });
   });
 
@@ -114,7 +113,7 @@ describe('stackingOffset', () => {
     expect(last.z).toBeCloseTo(40);
   });
 
-  it('returns no offset without a stacking strategy', () => {
-    expect(stackingOffset(undefined, 0, 3)).toBe(NO_OFFSET);
+  it('returns no offset for an empty stack', () => {
+    expect(stackingOffset(undefined, 0, 0)).toBe(NO_OFFSET);
   });
 });

packages/tabletop/src/stacking.ts

@@ -27,8 +27,8 @@ export const NO_OFFSET: StackOffset = { x: 0, y: 0, rotation: 0, z: 0, tilt: 0 }
 
 /**
  * Compute the offset/rotation for the piece at `index` of a `stackSize`-piece
- * stack, given the route's stacking strategy. Returns `NO_OFFSET` when there's
- * no curve, no tilt, and no z profile, or the stack is empty.
+ * stack, given the route's stacking strategy. Returns `NO_OFFSET` when the
+ * stack is empty. Every placed part gets a default 1° tilt unless overridden.
  */
 export function stackingOffset(
   stacking: Stacking | undefined,
@@ -45,8 +45,8 @@ export function stackingOffset(
 
   // `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);
+  // strategy doesn't specify a tilt.
+  const tilt = stacking?.tilt ?? 1;
 
   // 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.