refactor(editor): extract beatOf/timeOf tempo-map converter (Phase A1)

[ChrisBeWithYou]

Workspace-shell time-model foundation — Phase A1 (charrette §1.1/§1.10). First step of the beat-primary time model; a pure refactor with no behaviour change, parallel-safe with the open feature-PR backlog (barely touches chrome).

What

The musical-beat ⇄ seconds math that _makeTimeRemap and snapTime each computed inline is now ONE named, shared @pure:beat-converter pair, so every musical-time quantity (ruler, snap, loop, playhead, and — from A2 on — notes) reads a single implementation.

• beatOf(beats, t) — seconds → fractional beat. Exact inverse of timeOf within a grid gap; before the first / after the last beat it extrapolates along that gap's local tempo, so the pair stays invertible outside the grid too.
• timeOf(beats, β) — fractional beat → seconds, the mirror.
• < 2 beats ⇒ both are the identity (no tempo map → notes stay seconds-primary, §1.3.4). Non-finite input and zero-width gaps guarded.

_makeTimeRemap now routes interior times through the converter — remap is timeOf(new, beatOf(old, t)) within the grid — while keeping its legacy constant-shift on the tails, so the extraction is behaviour-identical across the whole domain. snapTime snaps in the beat domain: timeOf(round(beatOf(t)·subs)/subs).

No note-model change — that is A2. This PR only extracts + names the converter and rewires the two existing callers to it.

Tests — tests/beat_converter.test.js (13)

• inverse both directions (exact, 1e-9)
• the converter reproduces the pre-refactor _makeTimeRemap in the interior (the charrette identity)
• the shipped _makeTimeRemap and snap math match the pre-refactor code across the whole domain — the no-behaviour-change proof
• both-tail extrapolation, the <2-beats identity, adversarial guards (empty / null / NaN / Inf / zero-width)

beatOf/timeOf do not exist on main, so the suite fails on main (would-fail-on-main).

tests/tempo_ride_parts.test.js (+3): its hand-assembled sandbox extracts the real _makeTimeRemap, which now needs the converter block in scope. Full JS suite green.

🤖 Generated with Claude Code

https://claude.ai/code/session_013kpQfrmvrUHsZ8asuPnuW4

Summary by CodeRabbit

• Bug Fixes

  • Improved time snapping and tempo-map remapping consistency by standardizing conversions through a shared beat/seconds logic.
  • Preserves legacy tail/endpoint behavior while improving accuracy across interior gaps and boundary cases (including degenerate or irregular timing inputs).

• Tests

  • Added/expanded coverage for converter inverse properties, remap equivalence, and snapping invariants (ties, idempotence, subdivision limits, extrapolation).
  • Updated TempoMap pipeline test setup to include the new beat-converter logic in scope.

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Review details

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 540c13c7-7c97-49ab-9042-e8906def08af

📥 Commits

Reviewing files that changed from the base of the PR and between 7511056 and 21934e7.

📒 Files selected for processing (2)

• CHANGELOG.md
• screen.js

📝 Walkthrough

Walkthrough

This PR extracts beat/time conversion into shared pure helpers in screen.js, refactors remapping and snapping to use them, adds a dedicated converter test suite, updates one pipeline test scope, and records the change in the changelog.

Changes

Beat converter extraction

Layer / File(s)	Summary
Pure converter implementation screen.js	Adds beatOf(beats, t) and timeOf(beats, beat) for seconds↔fractional-beat conversion with extrapolation and degenerate/non-finite input guards.
Remap and snap refactor screen.js	Rewrites _makeTimeRemap to route interior conversions through timeOf(newBeats, beatOf(oldBeats, t)) while preserving tail constant-shift behavior; refactors snapTime to round in beat-domain via the converter.
New beat converter test suite tests/beat_converter.test.js	Adds a runner extracting the converter/remap logic from screen.js, legacy reference implementations, and tests for inverse properties, remap equivalence, snap equivalence, extrapolation, degenerate grids, and robustness edge cases.
Existing test update and changelog tests/tempo_ride_parts.test.js, CHANGELOG.md	Adds the beat-converter block to the pipeline test's evaluation scope and documents the refactor in the changelog.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Possibly related PRs

• got-feedBack/feedBack-plugin-editor#110: Updates the same tempo-ride pipeline/test to include the beat-converter pure block so the tempo remap path works with the refactored beatOf/timeOf conversions.

🚥 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 clearly and concisely summarizes the main refactor: extracting the beatOf/timeOf tempo-map converter.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/editor-beat-converter

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