Per-section sync 4/5: bake the piecewise warp into note timestamps (offline, with round-trip) #25

Effort: L

Part of #8. Depends on sub-issue 2 (data-model decisions) and sub-issue 3 (alignment results).

Apply the per-section results once, offline, consistent with the rhythm-game model: fixed committed timestamps, no live warping at play time.

Scope

• Bake the per-section (factor, offset) / anchors into note timestamps, per the boundary handling and beat-grid decisions from sub-issue 2.
• Reconcile the ebeats grid as decided (re-derived or separately authored).
• Allow applying or skipping individual sections, so a low-confidence section can be left untouched while others are applied.
• Make the change survive the full save / build / XML round-trip, including any schema addition (and, if a pack-format key is involved, only after it has landed in feedpak-spec).

Acceptance criteria

• Applying a subset of sections leaves the others' timestamps bit-for-bit unchanged.
• Round-trip (save, reload, build, reload) preserves the warped timestamps and the chosen beat grid.
• Undo/restore path exists (the editor already has history concerns; state what it uses), so a bad apply is recoverable.
• The uniform model's do-no-harm guard from sub-issue 1 still passes.

Out of scope

The alignment engine (sub-issue 3) and the review UI (sub-issue 5).