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).

board_anim.sx

@@ -144,18 +144,25 @@ BoardAnim :: struct {
     active: bool;
     elapsed: f32;
     move: AnimMove;
+    // Highest 1-based cascade round whose ascending combo cue has already played,
+    // so the frame loop's per-round SFX is edge-triggered: a round's cue fires once,
+    // when its clear begins, never re-fired every frame. Reset whenever a move
+    // (re)starts; advanced by the frame loop as rounds clear.
+    cascade_fired: s64;
 
     init :: (self: *BoardAnim) {
         self.active = false;
         self.elapsed = 0.0;
         self.move.legal = false;
         self.move.rounds = List(AnimRound).{};
+        self.cascade_fired = 0;
     }
 
     begin :: (self: *BoardAnim, m: AnimMove) {
         self.move = m;
         self.elapsed = 0.0;
         self.active = true;
+        self.cascade_fired = 0;
     }
 
     // Total wall-clock length: the swap segment plus a clear+fall pair per round.
@@ -190,6 +197,22 @@ BoardAnim :: struct {
     }
 }
 
+// Per-round cascade-cue timing (P10.10): how many cascade rounds have BEGUN their
+// clear (pop) by `elapsed`, on the SAME swap→(clear,fall)* timeline `phase` walks.
+// Round k (0-based) starts clearing at SWAP_ANIM_DUR + k*(CLEAR_ANIM_DUR +
+// FALL_ANIM_DUR), so this is the count of rounds whose ascending combo cue should
+// have fired by now (clamped to the move's round count). The frame loop diffs it
+// against `BoardAnim.cascade_fired` to play one cue per newly-cleared round. Pure +
+// headless so the per-round playback is snapshot-testable without audio.
+cascade_rounds_started :: (elapsed: f32, num_rounds: s64) -> s64 {
+    if num_rounds <= 0 { return 0; }
+    if elapsed < SWAP_ANIM_DUR { return 0; }
+    seg := CLEAR_ANIM_DUR + FALL_ANIM_DUR;
+    started := cast(s64) ((elapsed - SWAP_ANIM_DUR) / seg) + 1;
+    if started > num_rounds { return num_rounds; }
+    started
+}
+
 // Input gate: the board accepts a new swipe/tap gesture only when no move
 // animation is in flight. The view checks this at gesture START (mouse_down),
 // not at commit (mouse_up), so a gesture begun while a timeline is playing never