feat(editor): add measure signature readout

[ChrisBeWithYou]

Summary

• add a compact toolbar readout for the current measure and time signature
• make the readout follow the cursor in note view and the selected sync point in Tempo Map view
• add pure helper coverage for cursor-following, selected-sync-point, denominator fallback, and empty-grid states

Testing

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

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/editor-measure-signature-readout

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