writeMidiFile: emit PART DRUMS instrument tracks

[elicwhite]

Adds drum emission to writeMidiFile:

• buildDrumTrack: groups one or more ParsedTrackData entries (one per
  difficulty) into a single PART DRUMS MIDI track. Emits trackName,
  delegates per-difficulty notes to emitDrumNotes, collects and dedupes
  instrument-wide star power (MIDI 116) / solo (MIDI 103) / activation
  lanes (MIDI 120), emits flex lanes (MIDI 126/127 with per-difficulty
  LDS velocity), passes through per-track text events and
  unrecognizedMidiEvents from the first difficulty.

• emitDrumNotes: per-difficulty note emission with full modifier support:
  base drum notes (MIDI 96..100 expert / 84..88 hard / 72..76 medium /
  60..64 easy); double-kick as MIDI 95 only, emitted AFTER regular kick
  at the same tick (YARG insertion order quirk); tom markers (MIDI
  110/111/112) only in fourLanePro; accent (velocity 127) / ghost
  (velocity 1) encoding; one flam marker (MIDI 109) per group; 5-lane
  green+cymbal remapped to offset 4 (orange pad); blue-at-same-tick-as-
  green+cymbal remapped to offset 5 so fiveLane detection succeeds;
  fourLanePro green+tom fallback to offset 5 when a conflicting
  green+cymbal exists at the same tick across difficulties; disco-flip
  state-transition text events ([mix <diff> drums0[d|dnoflip]]).

• fourLanePro sentinel greenTomMarker: when drumType=1 but no tom
  markers were emitted (all yellow/blue defaulted to cymbal, green-toms
  used offset-5 encoding), emit a MIDI 112 at a safe tick so scan-chart's
  drumType detection still picks fourLanePro.

• [ENABLE_CHART_DYNAMICS] text event at tick 0 when any accent or ghost
  was emitted.

• computeLengthOverrides: prevents scan-chart's trimSustains from
  collapsing adjacent short-sustain drum-note chains by attributing the
  combined chain length to the first note.

• Shared helpers: addNoteOnOff, addNoteOnOffWithChannel (with
  zero-length seq-pairing to survive finalizeMidiTrack's sort),
  deduplicateSections, instrumentTrackNames table.

Fret tracks (PART GUITAR, GHL) and vocal tracks (PART VOCALS, HARM1/2/3)
remain no-ops until the follow-up PRs land — the writeMidiFile entry
iterates trackData but skips any non-drum instrument group.

[elicwhite]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• Add ChartDocument + writeChartFolder orchestrator #78 : 2 dependent PRs (#79 , #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 👈 (View in Graphite)
• 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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