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).
