Consolidate song metadata onto parsedChart.metadata

[elicwhite]

Widens ParsedChart.metadata to Partial & {
extraIniFields?: Record<string, string>}, and has parseChartAndIni merge
[Song]-section values + song.ini into it (ini wins, matching the
"song.ini is authoritative, [Song] is a legacy overlap" stance). Unknown
ini key/value pairs move from ParseChartAndIniResult.iniUnknownValues
onto parsedChart.metadata.extraIniFields for round-trip writing.

ParseChartAndIniResult.iniMetadata and iniUnknownValues are removed --
the same data is now reachable via result.parsedChart.metadata.
iniFolderIssues and iniMetadataIssues stay (they're diagnostics, not
duplicate data).

scanChart's diff-against-default checks now read the merged metadata via
parseResult.parsedChart.metadata; "set" is defined as "present AND not
equal to the default". This also fixes a pre-existing inconsistency
where chart.delay was taken from ini but chart.chart_offset was taken
from [Song] -- both now come from the merged metadata with ini winning.

Also derives defaultIniChartModifiers as a projection of defaultMetadata
so the 8 parse-behavior defaults can't drift from the 40-field source
of truth.

[elicwhite]

• perf: swap writer's spread-with-deltaTime for Object.assign #81
• perf: midi-file Writer uses preallocated Uint8Array #80
• perf: patch midi-file writeBytes to in-place push loop #79
• Add ChartDocument + writeChartFolder orchestrator #78
• 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
• Extract shared natural-HOPO helpers into natural-hopo.ts #73
• Move hasLyrics/hasVocals/hasForcedNotes from ParsedChart to ScannedChart (state-derived) #67
• Consolidate song metadata onto parsedChart.metadata #66 👈 (View in Graphite)
• master

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