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)
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