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