Add ChartDocument + writeChartFolder orchestrator

[elicwhite]

Bundles a ParsedChart with non-chart files (audio, album art, extras)
into a single value, and provides the write-side companion to
parseChartAndIni's input shape:

files (in)                    ChartDocument (out)
----------------              -----------------------
notes.chart or notes.mid   →  parsedChart
song.ini                   →  parsedChart.metadata
song.ogg, album.png, ...   →  assets (passthrough)

writeChartFolder produces the reverse transformation:

• notes.chart or notes.mid from writeChartFile / writeMidiFile based on
  parsedChart.format
• song.ini from writeIniFile(parsedChart.metadata) — chart_offset is
  naturally skipped since writeIniFile only emits keys in defaultMetadata
• every entry in doc.assets, in order

No separate metadata field on ChartDocument — it lives on
parsedChart.metadata (the consolidated shape produced by parseChartAndIni).

Tests: 9 cases covering format selection, ini content (chart_offset not
leaked, extraIniFields preserved), asset passthrough + ordering, and
full round-trip through parseChartAndIni for both .chart and .mid with
assets.

442 total scan-chart tests passing.

[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 ) 👈 (View in Graphite)
• 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

This stack of pull requests is managed by Graphite. Learn more about stacking.

[elicwhite]

There are lots of references in the codebase to

{ fileName: string; data: Uint8Array }

In a separate PR before this in the stack, create a type in interfaces.ts called File, and update every callsite through the codebase to reference File or File[]. Then update this PR to use that instead of ChartAsset.

[elicwhite]

This should verify that there aren't any other song.ini or .chart/.mid files passed in assets. If there are, they should be removed first.

@Geomitron, I'm making a choice to just overwrite, because I think it'll be pretty common that people don't remove these to pass in just their assets.