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).

assets/audio/LICENSE.txt

@@ -0,0 +1,9 @@
+m3te sound effects
+==================
+
+clear.wav — "confirmation_001" from Kenney's Interface Sounds pack
+(www.kenney.nl), licensed CC0 1.0 (public domain):
+https://creativecommons.org/publicdomain/zero/1.0/
+
+Converted to mono 44.1 kHz signed-16-bit PCM WAV (afconvert) — the format
+iOS System Sound Services loads directly via audio.sx.