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-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`) ✅