feat(editor): add tempo map signature control

[ChrisBeWithYou]

Summary

• add a compact Sig control in Tempo Map mode for direct selected-measure beat-count edits
• keep the existing bracket hotkeys/context menu paths backed by the same undoable grid command
• cover beat-count resubdivision with a focused Node test

Notes

• this edits the persisted beat-grid measure count only; denominator support needs the later song timeline/time_signatures model integration

Testing

• node --check screen.js
• node tests/loop_region.test.js
• node tests/pending_view.test.js
• node tests/tempo_map_bpm.test.js
• node tests/tempo_map_timesig.test.js

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• Commit unit tests in branch editor-tempo-timesig

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[byrongamatos]

Review pass (Claude Code) — draft, stacked on #49. Its own change (the Sig beat-count control via _tempoSetBeatsPerMeasurePure + TempoGridCmd) is correct: it replaces a measure's interior with count-1 evenly-spaced beats between the fixed downbeats (constant tempo, right beat count), count=1 yields a bare 1-beat measure, and the final measure uses duration as the closing boundary. Interior beats are tagged measure: -1, which every consumer treats identically to a <= 0 sub-beat, and _tempoRenumberMeasures leaves them untouched. node --check + all JS tests green. No change needed. (Codex second-opinion was unavailable — rate limit — so this is manual + test-backed.)

[Copilot]

Pull request overview

Adds a compact time-signature (“Sig”) control to Tempo Map mode, wiring it through the existing undoable tempo-grid editing path and adding a focused Node-based test for the pure resubdivision helper.

Changes:

• Render/update a new “Sig” input in the toolbar during Tempo Map mode, and expose window.editorSetTempoSignature.
• Extract a pure helper _tempoSetBeatsPerMeasurePure (with @pure: markers) and route the existing grid command through it.
• Add tests/tempo_map_timesig.test.js to validate resubdivision behavior and clamping.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
screen.js	Adds the Sig UI control + display updates, exposes editorSetTempoSignature, and extracts/tests a pure resubdivision helper for beat-grid time-signature edits.
tests/tempo_map_timesig.test.js	New Node test covering beat-count resubdivision, clamping, and final-measure duration boundary behavior for the new pure helper.

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