feat(tabletop): add tilt and zStart/zEnd stacking options

Replace the per-part lift with a local Y-axis tilt that fans the stack,
and add zStart/zEnd to ramp the stack's height across the curve so it
arches in 3D. Update the poker deck and docs accordingly.

docs/bgm-format.md

@@ -359,6 +359,9 @@ layout:
       limit: 5
       align: center
       steps: 4
+      tilt: 0.1
+      zStart: 0
+      zEnd: 30
 ```
 
 - `curve` — an SVG path string to spread the content along, relative to the
@@ -368,6 +371,13 @@ 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 radians 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.
+- `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`.
 
 #### positioning process
 
@@ -377,6 +387,10 @@ layout:
    `step length × (# of parts − 1)` on the curve.
 3. **Place each part.** Part `#0` is at the start, the last part at the end,
    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.
 
 ### Edge cases
 

docs/bgm-tabletop-plan.md

@@ -118,10 +118,12 @@ interface GameState {
 
 ### 7. Stacking (`stacking.ts`) ✅
 
-- `useStacking(route.stacking, index, stackSize)` → `{ offset, rotation }`.
+- `useStacking(route.stacking, index, stackSize)` → `{ x, y, rotation, z, tilt }`.
 - Implements the format's positioning process (`bgm-format.md` §4): step
   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.
 - Curve length from an SVG path string (small helper; no new dep).
 
 ### 8. Public API (`index.ts`) ✅
@@ -148,7 +150,8 @@ consumers share them (see Open decisions).
 
 - `state.ts` — derived render state: enabled surfaces, route matching,
   candidate selection, stacking index/stackSize.
-- `stacking.ts` — positioning process: step length, alignment, limit.
+- `stacking.ts` — positioning process: step length, alignment, limit, z ramp,
+  tilt.
 - `setup.ts` — seeding + bare-type expansion.
 - `mount.ts` — mount tree resolution (table/hud/child, children refs).
 - `partView.tsx` — geometry from a part def (size/fillet/crop), sprite UVs.

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, 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. `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 per-part fan about the card's local Y (long) axis. `PartPlacement` consumes it.
 
 ## 5. usage