[SINGSYNC] HKA-79 rough line timing generator from markers + lyrics

[hkalbertkim]

Summary

• implement rough line timing generator service (syncLineTiming) using vocal onsets + phrase gaps + cached lyric lines
• add v1 heuristic for segment creation and line allocation (M>=N direct mapping, M<N proportional distribution, marker-missing fallback)
• persist alignment.mode and alignment.line_timings[] into sync.json with per-line confidence and derived_from
• wire timing generation into /api/prepare after marker extraction (background prepare stage, no mid-song work)
• add CLI helper to backfill line timings for cached songs
• document CLI usage in README

Linear

• HKA-79

Notes

• This branch includes prerequisite sync foundation commits from HKA-77/HKA-78 because main did not yet contain them.

Verification

• (cd server && npm exec tsc --noEmit)
• ./scripts/extract-line-timings.sh fixture_test

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Project	Deployment	Actions	Updated (UTC)
sing-sync	Ready	Preview, Comment	Feb 14, 2026 2:28pm