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