feat(editor): add tempo signature denominator control

[ChrisBeWithYou]

Summary

• add a Tempo Map denominator selector for selected-measure signatures
• display selected measures as numerator/denominator labels on the tempo grid
• keep numerator edits using the selected denominator in status feedback
• add unit coverage for denominator normalization and downbeat-only edits

Testing

• node --check screen.js
• node tests/tempo_map_timesig.test.js
• node tests/tempo_map_bpm.test.js
• all tests/*.test.js via node

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✨ Finishing Touches

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• Commit unit tests in branch feat/editor-tempo-signature-denominator

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[byrongamatos]

Review pass (Claude Code) — stacked on #60; extends #50's numerator work (in its base). Own logic is correct: _tempoNormalizeDenominatorPure clamps to {2,4,8,16} (default 4), _tempoSetDenominatorOnBeatsPure stores beat.den on the downbeat via the undoable TempoGridCmd (doesn't touch beat times), and the grid label + numerator status message read it back consistently. node --check + all JS tests green (6 timesig tests).

Note (scope, not a bug): beat.den is display-only — it isn't serialized to the manifest, so a denominator change is lost on save/reload. That matches #50's own note ('denominator support needs the later song timeline/time_signatures model integration'), so it's the intended incremental split; the model/persistence side is the follow-up. Only other flags are stack-inherited (built on the draft #50 tempo work + the pre-fix #55 drum-shortcut base). (Codex rate-limited until ~1 AM — manual + test-backed.)