writeChartFile: emit instrument track sections

[elicwhite]

Completes writeChartFile with per-track section emission:

• [ExpertSingle], [HardDrums], etc. named from instrument + difficulty.
• Drum tracks: base notes (N 0..4 or 0..5 for 5-lane), cymbal markers
  (N 66/67/68) only in fourLanePro, accent (N 34..38) and ghost (N 40..44)
  markers matching the emitted note's eventType, one N 109 flam per group,
  double kick as N 32 only.
• 5-lane round-trip: greenDrum+cymbal emits as N 4 (orange) while plain
  greenDrum stays at N 5; blueDrums coinciding with greenDrum+cymbal emit
  as N 5 to keep the parser's drumType detection on fiveLane.
• Fret/GHL tracks: notes via the 5-fret and 6-fret maps, tap as N 6,
  forceUnnatural (N 5) when the note's hopo/strum flag disagrees with
  the natural-HOPO state.
• Star power S 2, activation lanes S 64, flex lanes S 65/66, versus
  phrases S 0/1, solo sections as E 'solo'/'soloend' (tick + length - 1
  for soloend to round-trip).
• Per-track text events (including disco flip mix markers).
• Disco-flip state transitions per difficulty from red/yellow drum
  disco/discoNoflip flags → 'mix drums0[d|dnoflip]' text events.
• Coda events: if no 'coda' in unrecognizedEvents, synthesize from
  drumFreestyleSections where isCoda=true.
• hasForcedNotes backstop: if hasForcedNotes is set but no forceUnnatural
  emitted, append N 5 0 at a vacant tick in the first fret track to
  preserve the flag on round-trip.

[elicwhite]

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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
• Add writeMidiFile() with TEMPO/EVENTS/unrecognized-track emission #72
• writeChartFile: emit instrument track sections #71 👈 (View in Graphite)
• 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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