Use AudioContext lookahead scheduler for MIDI playback

[jeromepotter]

Motivation

• Replace brittle per-note setTimeout scheduling with the Web Audio clock to get microsecond-precise timing.
• Make MIDI playback robust when the window is dragged, CPU is overloaded, or the tab is backgrounded by using the AudioContext clock.

Description

• Replaced per-note setTimeout timers with an AudioContext-backed lookahead scheduler that uses audioContext.currentTime and performance.now() when calling midiOutput.send(message, timestamp).
• Accumulate and sort note on/off entries into an events array and schedule them in a short lookahead loop using LOOKAHEAD_MS and SCHEDULE_AHEAD_TIME.
• Added audioContext initialization/resume logic and new timer handles schedulerTimerId and finishTimeoutId, and updated stopPlayback to cancel those timers and send silence CCs.
• Made playSequence asynchronous and awaited it from generateMusic to ensure scheduler setup completes before signaling success.

Testing

• No automated tests were executed for this change.

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Codex Task

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