fix(editor): snap dragged barlines to chart-time onsets (fixes #254)

[ChrisBeWithYou]

Fixes #254 (Byron's review catch on #249).

_tempoMapOnDragMove snapped a dragged barline to the nearest detected onset using _ensureOnsets() (buffer time) against rawT (chart time). When the recording is shifted (S.audioShift != 0) the two time bases diverge, so the barline snapped to the un-shifted attack instead of the heard one.

Fix: use _ensureOnsetsShifted() — the chart-time onsets the Suggest-fit path already uses (tempo.js ~L691). Pre-existing since the onset-snap-on-drag change (#235); low impact (only bites with shifted audio), undoable, but a wrong snap target.

• Drops the now-unused _ensureOnsets import from tempo.js.
• tempo_onset_snap.test.mjs's source-text assertion updated to _ensureOnsetsShifted.
• 146 JS green, ESLint 0 errors, routes.py untouched.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • Improved barline dragging with Snap = Onset so snapping stays correct when audio and chart timing are offset.
  • Onset snapping now uses shift-corrected chart-time onset positions, avoiding snaps to misaligned attacks.
• Tests
  • Updated the tempo-map onset-snap drag test to verify snapping behavior against chart-time onset references.

[coderabbitai]

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 86985678-2426-4e66-b5de-2c4b2d5e7569

📥 Commits

Reviewing files that changed from the base of the PR and between c2116da and 2be6164.

📒 Files selected for processing (2)

• src/tempo.js
• tests/tempo_onset_snap.test.mjs

🚧 Files skipped from review as they are similar to previous changes (1)

• tests/tempo_onset_snap.test.mjs

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📝 Walkthrough

Walkthrough

Tempo-map barline dragging with onset snapping now compares chart-time positions against shift-corrected onsets. The unused unshifted onset import is removed, and the related guard test is updated.

Changes

Tempo onset snapping

Layer / File(s)	Summary
Use shifted onsets for drag snapping src/tempo.js, tests/tempo_onset_snap.test.mjs	The drag handler now calls _ensureOnsetsShifted() for onset snapping, removes the unused _ensureOnsets import, and updates the source-inspection assertion.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related PRs

• got-feedBack/feedBack-plugin-editor#235: Introduced the tempo-map barline onset-snapping logic changed here.
• got-feedBack/feedBack-plugin-editor#236: Added the shifted-onset handling used by this fix.
• got-feedBack/feedBack-plugin-editor#144: Modified related onset-snapping logic in the same implementation area.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title is concise and accurately summarizes the main change: snapping dragged barlines to chart-time onsets.
Linked Issues check	✅ Passed	The change replaces buffer-time onsets with _ensureOnsetsShifted() in _tempoMapOnDragMove, matching issue #254's fix.
Out of Scope Changes check	✅ Passed	The PR stays focused on the onset-snap bug fix and the corresponding test update, with no unrelated changes.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

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🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/editor-tempo-snap-timebase

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