perf: midi-file Writer uses preallocated Uint8Array

[elicwhite]

Replace Writer's backing Array with a growable Uint8Array +
manual cursor. Benefits:

• Raw bytes instead of boxed JS numbers — ~8× less memory per byte
  and much less GC pressure.
• writeBytes on a Uint8Array source is a single .set() call instead
  of a push loop.
• writeVarInt no longer allocates a temp Array to hold the bytes
  then .reverse()s it — the 1-5 byte tail is written directly into
  the buffer.

API shape preserved for callers:

• Inner sub-writers expose .used() which returns a subarray view.
• Top-level writeMidi returns the used subarray; scan-chart's
  new Uint8Array(writeMidi(midiData)) still works (copies the
  subarray into a dedicated Uint8Array), and the few other direct
  callers in spotify-clonehero-next that wrap the result in
  Uint8Array also still work.

Measured on writer autoresearch bench (2000 charts, 8 workers):
previous (post-writeBytes fix): 6.721 ms mean, 25.263 ms p99, 77 ms max, 5.496s wall
this patch: 5.710 ms mean, 22.576 ms p99, 42 ms max, 5.132s wall
delta: -15.0% mean, -10.6% p99, -45% max, -6.6% wall

0 hash mismatches, 442/442 tests still green.

[elicwhite]

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Learn more

• perf: swap writer's spread-with-deltaTime for Object.assign #81
• perf: midi-file Writer uses preallocated Uint8Array #80 👈 (View in Graphite)
• perf: patch midi-file writeBytes to in-place push loop #79
• Add ChartDocument + writeChartFolder orchestrator #78 : 1 other dependent PR (#87 )
• Add same-format round-trip integration tests #77
• writeMidiFile: emit PART VOCALS / HARM1 / HARM2 / HARM3 tracks #76
• writeMidiFile: emit PART GUITAR / GHL instrument tracks #75
• writeMidiFile: emit PART DRUMS instrument tracks #74
• Add writeMidiFile() with TEMPO/EVENTS/unrecognized-track emission #72
• writeChartFile: emit instrument track sections #71
• Add writeChartFile() with Song/SyncTrack/Events/unrecognized-section emission #70
• Add writeIniFile() for song.ini emission #69
• Add createEmptyChart() counterpart to parseChartAndIni #68
• Add shared File type in interfaces.ts #86
• master

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