P6.3: per-gem idle/select/land/clear animations (sx, iOS sim)

New gem_anim.sx adds a purely-visual per-gem pose set driven by a single
animation clock: a calm always-on idle breath (scale-pulse + bob, per-gem
phase, ramped in from rest), a selection pop, a landing squash-bounce, and
a clear pop. BoardView draws every settled gem through gem_pose_at /
gem_pose_frame; the move timeline (P6.1) and FX (P6.2) are untouched and the
input-lock semantics are unchanged (idle never locks input).

Determinism: the idle is always-on, so main reads M3TE_ANIM_TIME=<seconds>
to freeze the clock at a chosen phase (t==0 == the resting board, so the
pre-P6.3 goldens reproduce) and M3TE_SELECT=<cellIndex> to force a selection
for capture. tests/gem_pose.sx locks the t==0-rest invariant and the reaction
envelopes headlessly (fails if the idle ramp is dropped).

Goldens (deterministic capture): p6_idle_t0 (resting), p6_idle_mid (pinned
mid-breath), p6_select (selection pop on cell 3,3). Purely visual: no change
to model/score/moves/hit-testing.

README.md

@@ -57,3 +57,30 @@ The screenshot should match `goldens/p0_quad.png` (a centered orange quad over a
 blue clear), modulo the status-bar clock — pixel-exact equality is not required.
 A tap on the quad flips its color (orange ↔ green); see
 `goldens/p0_input_before.png` / `goldens/p0_input_after.png`.
+
+### Deterministic animation capture (P6.3)
+
+The per-gem idle loop (`gem_anim.sx`) is always-on, so a plain screenshot is
+time-dependent. Two environment variables pin the visual state so the board can
+be captured reproducibly. The simulator forwards any `SIMCTL_CHILD_*` variable to
+the launched app, so prefix them on the `simctl launch`:
+
+- `M3TE_ANIM_TIME=<seconds>` freezes the animation clock at that phase. **`t=0`
+  is the resting board** — every gem sits at its static pose, so the pre-P6.3
+  goldens reproduce unchanged. A larger `t` (e.g. `1.0`) shows the mid-breath
+  idle deformation. The select/land reactions read this same pinned phase.
+- `M3TE_SELECT=<cellIndex 0..63>` (= `row*8 + col`) force-selects a cell at
+  startup, so the selection highlight + pop can be captured without a tap.
+
+```bash
+# Resting board (idle at rest): goldens/p6_idle_t0.png
+SIMCTL_CHILD_M3TE_ANIM_TIME=0   xcrun simctl launch booted co.swipelab.m3te
+# Mid-breath idle: goldens/p6_idle_mid.png
+SIMCTL_CHILD_M3TE_ANIM_TIME=1.0 xcrun simctl launch booted co.swipelab.m3te
+# Selection pop on cell (3,3): goldens/p6_select.png
+env SIMCTL_CHILD_M3TE_ANIM_TIME=0.17 SIMCTL_CHILD_M3TE_SELECT=27 \
+    xcrun simctl launch booted co.swipelab.m3te
+```
+
+With no variable set the game runs fully live (the clock advances by
+`delta_time`). `tests/gem_pose.sx` locks the `t==0`-rest invariant headlessly.