Audio: clean up event listeners and buffer view on destroy

[kwigbo]

The Audio constructor binds three window-level event listeners (keydown / click / touchend) to drive startPlayback. They're only removed when startPlayback() fires; if the Emulator is destroyed before the user produces a gesture — or if an embedder recreates the Emulator multiple times before that happens — the listeners stayed attached to a stale Audio instance forever.

Audio also holds a TypedArray view (this.buffer) into the wasm heap region returned by _get_audio_buffer_ptr. After Emulator destroy() calls _emulator_delete + _free, that backing memory is gone, and any held reference to the buffer view points at freed memory.

Add Audio.destroy(): removes the gesture listeners and nulls out the buffer view. Call it from Emulator.destroy() before _emulator_delete so the cleanup happens while the wasm pointer is still valid.

[Copilot]

Pull request overview

This PR improves lifecycle cleanup in the browser demo’s audio subsystem to prevent stale window-level gesture listeners and to avoid retaining a TypedArray view into freed wasm memory after an Emulator instance is destroyed.

Changes:

• Call this.audio.destroy() from Emulator.destroy() before deleting the wasm emulator instance.
• Add Audio.destroy() to remove the keydown/click/touchend gesture listeners (if still installed).
• Clear Audio’s wasm-backed buffer view on destroy to avoid referencing freed memory.

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