P10.10: play one ascending combo cue per cascade round

For combos, play a sound for each match ascending (Candy-Crush cascade run):
as a chain resolves, EACH successive round plays the next higher cue
(combo1, combo2, … clamped at combo5) instead of a single combo cue keyed
to the final cascade depth at commit.

- board_anim.sx: add `BoardAnim.cascade_fired` (edge-trigger high-water mark,
  reset on init/begin) and the pure `cascade_rounds_started(elapsed, n)` helper
  — how many rounds have begun clearing on the swap→(clear,fall)* timeline.
- main.sx: in the frame loop, diff `cascade_rounds_started` against
  `cascade_fired` and play one ascending cue per newly-cleared round, once each,
  gated on a real multi-round chain (rounds >= 2). Additive; never touches
  board/score/move state.
- board_view.sx: drop the single `sfx_cascade(final depth)` at commit; keep
  `sfx_swap` / `sfx_match` (and win/lose) exactly as before.
- tests/cascade_rounds.sx: headless snapshot of the per-round timing + the
  ascending combo1..combo5 run with the combo5 clamp.

Sim (M3TE_FX=11, depth-5): log show shows combo1→combo2→combo3→combo4→combo5
at successive timestamps ~0.36s apart (= CLEAR+FALL per-round spacing).

tests/expected/cascade_rounds.exit

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+0

tests/expected/cascade_rounds.stdout

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+== per-round cascade cue timing ==
+-- started-count across a 5-round chain --
+e=0.00 -> 0
+e=0.10 -> 0
+e=0.20 -> 1
+e=0.50 -> 1
+e=0.55 -> 2
+e=0.90 -> 3
+e=1.30 -> 4
+e=1.70 -> 5
+e=5.00 -> 5
+-- ascending per-round run --
+round 1 -> [sx] audio: cue combo1
+round 2 -> [sx] audio: cue combo2
+round 3 -> [sx] audio: cue combo3
+round 4 -> [sx] audio: cue combo4
+round 5 -> [sx] audio: cue combo5
+-- non-cascade --
+1-round@end 1
+0-round@end 0
+-- deep-chain cue clamp --
+round 1 -> [sx] audio: cue combo1
+round 2 -> [sx] audio: cue combo2
+round 3 -> [sx] audio: cue combo3
+round 4 -> [sx] audio: cue combo4
+round 5 -> [sx] audio: cue combo5
+round 6 -> [sx] audio: cue combo5
+round 7 -> [sx] audio: cue combo5
+ok: one ascending combo cue per cascade round, clamped at combo5