P6.1: swap/clear/fall move tweens (sx, iOS sim)

Add a purely-visual animation timeline so the board no longer snaps on a
move. board_anim.sx records, on a value-copy of the pre-move board, the
swap and each cascade round's matched cells + per-column fall provenance,
then BoardView plays it over delta_time: the two swapped gems SLIDE between
cells (and ping out-and-back on an illegal swap), matched gems SCALE OUT,
and survivors FALL into place while refills drop in from above the grid.

The model stays authoritative: plan_and_commit still calls commit_swap on
the real board exactly as before, and the recording replays the identical
primitives from the identical cells + RNG state, so the timeline ends ON
the model's settled board. tests/anim_plan.sx is the determinism guard —
it asserts the committed board, score, moves, and the timeline's final
state all equal an independent commit_swap of the same move, that the
rounds are contiguous, and that an illegal swap records nothing and leaves
the board untouched. All pre-existing logic/cascade goldens stay green.

Evidence (sx-test-metal, iOS 26.0, time-sampled with temporarily-lengthened
durations; committed durations are the short production values):
goldens/p6_anim_swap.png  gems sliding between (5,4)/(6,4)
goldens/p6_anim_clear.png matched reds scaling out in row 4
goldens/p6_anim_fall.png  gems mid-fall with gaps + refill dropping in
goldens/p6_anim_after.png settled board == model (SCORE 30, MOVES 29/30)

tests/expected/anim_plan.exit

@@ -0,0 +1 @@
+0

tests/expected/anim_plan.stdout

@@ -0,0 +1,17 @@
+== legal swap: plan matches model ==
+model: legal true depth 1 score 30 moves 1
+plan:  legal true rounds 1 score 30 moves 1
+final==model true
+contiguous true
+final board:
+RRPBORRG
+PGPPOGRO
+YYBOPRYB
+GBYBYRGP
+OGBYRGOY
+BYRRPRBG
+YOYYROBB
+OROBPPRB
+== illegal swap: untouched ==
+legal false rounds 0 score 0 moves 0
+ok: animation layer leaves the model result unchanged