feat(editor): enable bpm edits in tempo map

[ChrisBeWithYou]

Summary

• keep the BPM input active in Tempo Map mode instead of greying it out
• edit the selected measure BPM through the existing tempo retime command path
• cover selected-measure BPM reshaping with a focused Node test

Testing

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

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• Commit unit tests in branch editor-tempo-bpm

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[byrongamatos]

Review pass (Claude Code) — draft, top of the loop/tempo stack (#47→#48→#49). Its own changes (per-measure BPM editing in Tempo Map mode via _tempoSetMeasureBpmPure + TempoMapCmd) are correct: even sub-beat respacing = constant BPM across the measure, downstream measures rigid-shift by dt (preserving their own tempo), startT fixed, gapMin floor prevents zero-width measures. node --check + all JS tests green; Codex clean. No change needed here. (Inherits the loop-strip/tone-lane layout item flagged on #48.)

[Copilot]

Pull request overview

This PR keeps the toolbar BPM input enabled while in Tempo Map mode and routes BPM edits to adjust the selected (or playhead-resolved) measure via the existing tempo remap/undo command path, with a focused Node test for the pure BPM-reshape helper.

Changes:

• Call updateBPMDisplay() during draw() so the BPM input reflects tempo-map selection/playhead context.
• Enable per-measure BPM editing in Tempo Map mode through editorSetBPM + TempoMapCmd.
• Add a pure helper (_tempoSetMeasureBpmPure) and a Node test that validates measure BPM reshaping and tail shifting behavior.

Reviewed changes

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

File	Description
screen.js	Keeps BPM input active in Tempo Map mode, resolves which measure to display/edit, and applies per-measure BPM edits via tempo remap history.
tests/tempo_map_bpm.test.js	Adds a Node test for the new pure tempo-map BPM reshaping helper block embedded in screen.js.

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