feat: keyboard input, Web Audio synthesis, and multiplayer demo
Compiler changes:
- emit_handler: DOM events (keydown/keyup/etc) route to document.addEventListener
- emit_handler: handler params (ev) pushed into scope to prevent .value suffix
- play_tone(freq, dur, type) and play_noise(dur, vol) builtins
- Web Audio runtime: _playTone (oscillator) and _playNoise (filtered noise)
- Reactive textContent: signal-dependent If expressions wrapped in DS.effect
Example updates:
- game-pong.ds: Arrow Up/Down for paddle, Space for pause/resume, paddle-hit sounds
- step-sequencer.ds: 4 audio trigger timers (kick=60Hz, snare=noise, hihat=noise, bass=110Hz)
Verification:
- All 136 tests pass, 45 examples compile
- Relay connects successfully, multiplayer sync confirmed
- Keyboard controls and pause toggle verified in browser
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