P8.1: minimal match/clear SFX via iOS System Sound Services (sx FFI)

Feasibility spike outcome: iOS audio from sx is feasible with no sx-library
change. System Sound Services is plain C, reached with the same `#foreign`
FFI uikit.sx already uses (UIApplicationMain / dlsym / CACurrentMediaTime);
AudioToolbox + CoreFoundation are linked per-target in build.sx.

Smallest viable SFX: one short CC0 clip (Kenney Interface Sounds, CC0 1.0)
played when a swap clears a match. Purely additive — audio.sx reads/writes
no score/board/move state; the wiring in board_view only adds a call.

- audio.sx: load clear.wav once, AudioServicesPlaySystemSound on clear
- board_view.sx: trigger sfx_clear() on a legal swap that clears (>=1 round)
- main.sx: allocate + init g_audio at boot
- build.sx: link AudioToolbox + CoreFoundation on iOS
- assets/audio/clear.wav (+ one-line CC0 credit in LICENSE.txt)

Verified: ios-sim build links; 18/18 tests pass; sim boot log shows
"[sx] audio: clear cue loaded" (AudioServicesCreateSystemSoundID succeeded,
asset shipped in the bundle and decoded).

main.sx

@@ -18,6 +18,7 @@
 #import "board_anim.sx";
 #import "board_fx.sx";
 #import "gem_anim.sx";
+#import "audio.sx";
 
 #run configure_build();
 
@@ -284,6 +285,14 @@ main :: () -> void {
     g_motion = xx context.allocator.alloc(size_of(GemMotion));
     g_motion.init();
 
+    // SFX (P8.1). Loads the one System Sound Services cue once; board_view
+    // plays it when a swap clears a match. Purely additive — never touches
+    // score/board/move state. On iOS the platform has already chdir'd to the
+    // bundle, so the cue's relative path resolves. No-op off iOS.
+    g_audio = xx context.allocator.alloc(size_of(GameAudio));
+    memset(xx g_audio, 0, size_of(GameAudio));
+    g_audio.init();
+
     // Deterministic-capture hooks: pin the animation clock and/or preselect a
     // cell so the always-on idle (and the select reaction) screenshot the same
     // way every time. No env set → fully live.