feat(editor): track regions — move a region by dragging its block

[ChrisBeWithYou]

The move step of the track-region layer: grab a track's region block in the Tracks (Parts) view and slide it along the timeline to reposition the track's content, as one undoable, bar-snapped step. Third in the region series — stacked on #333 (strip + selection), which is stacked on #332 (data model). Merge #332 → #333 → this.

This is PR 3a of a split: the charrette's PR 3 was commands + drag and the "Add Track from File" import driver; per the operability seat they're separated. The import driver (3b) is a follow-up and is blocked on a product decision (see below).

What it does

• src/region-commands.js (new): MoveRegionCmd. A notation region shifts the arrangement's contained notes; a drum region shifts S.drumTab's contained hits. Routes through S.history.exec (not the track-session commit() path, which never bumps editGen, so coverage/chord/lint memos would go stale). pitchPreserving so it passes the read-only-roll edit lock; the drum variant is songScope like every drum command.
• Time model (_regionRemapPure). A move preserves musical position, not wall-clock seconds — each note's whole interval remaps through the one tempo map (newTime = timeOf(beatOf(oldTime) + dBeat), sustain via its end-beat), mirroring TempoMapCmd. On a steady-tempo song this is just a clean shift; across a tempo change the part re-locks to the destination grid and a sustained note keeps its beat-length. Fast path: a constant/absent tempo collapses to one bit-exact seconds shift (no beatOf∘timeOf drift). dBeat === 0 is a no-op. Rollback restores a verbatim snapshot (the beat round trip is not bit-reversible), never inverts. (Move semantics reconciled with the rhythm-meter design lane against the seconds-primary reality; audio regions stay constant-dtime for the audio-region PR.)
• Bounded vs default. A bounded region rides its window startBeat with the move (and restores, incl. a move that re-sorts it past a neighbour in regions[]); the implicit default full-span region leaves regions[] untouched, carrying the move purely in its content — so untouched packs stay byte-identical.
• Drag wiring in parts-view.js: mousedown on a hit block arms the drag (transcription rows only — audio blocks still just select, pending the audio-region PR), a bar-snapped dashed ghost previews the drop (Alt = free), and mouseUp commits. Routed through two host hooks so the mouse.js move/up branches stay cycle-free.

Tests & gates

• tests/region_move.test.mjs (12): the pure time model (constant/varying tempo, fast path, snap), MoveRegionCmd exec→rollback deep-equality→redo for notation and drums, the bounded-region startBeat ride + restore, zero-delta no-op, and the real S.history.exec/undo/redo path.
• JS 298/0, lint 0 errors / 3 baseline warnings, routes.py untouched (no pytest).

Runtime-verified

Served through the real host and driven with Playwright (AC/DC — Back In Black, 9 tracks): dragging the Lead block commits the move (Moved "region:1" later), only the Lead track moves (Rhythm/Bass untouched), it snaps to bar lines (a sub-bar drag on this tempo-mapped song correctly no-ops), and Undo/Redo revert/re-apply exactly.

Follow-up (not this PR)

The "Add Track from File" import driver (drums-only GP into an existing project) is blocked on a product call: drums are a single session-wide S.drumTab — a project can hold only one drum part, so a new drum track isn't representable without extending the model to multiple drum tabs. Non-drum parts already add as N coexisting tracks. Flagged separately for the maintainer.

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📥 Commits

Reviewing files that changed from the base of the PR and between 25a3403 and dc4fa1b.

📒 Files selected for processing (8)

• CHANGELOG.md
• src/host.js
• src/main.js
• src/mouse.js
• src/parts-view.js
• src/region-commands.js
• src/region.js
• tests/region_move.test.mjs

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• Create PR with unit tests
• Commit unit tests in branch track-regions-p3

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