feat(editor): track regions — draw the region strip + region selection

[ChrisBeWithYou]

PR 2 of the track-regions arc — the region strip + selection

Stacked on #332 (the region data model) — merge #332 first. Base is track-regions-p1, so this shows only PR 2's diff.

Renders each track's content inside a region block in the Tracks (Parts) view, and adds region-level selection — the first visible step of the region layer. No move/trim yet.

What's here

• Each lane wraps its content (waveform / note silhouette) in a region block: a kind-colour spine down the left edge, a hairline border (accent when selected), and the region name when the lane is tall enough. Today every track has one full-span region, so the block spans the whole content; bounded blocks arrive with the move/trim PRs and need no renderer change (the span pure resolves them).
• Clicking a lane selects the region under the pointer (S.selectedRegionId) — the move/trim target a later PR acts on.
• New tested pure helpers in src/region.js: _regionTimeSpanPure, _regionBlockRectPure, _regionHitPure.

Rides the existing rAF Parts-view draw + timeToX; no new listeners or timers, no per-frame work added.

Tests

tests/region_layout.test.mjs (4), fails on main. Full suite green: JS 297/0 · lint 0 errors. Runtime-verified in the real host — region blocks render and the selection accent works.

Next

PR 3 is the import-into-existing-project driver (place / move / delete a region — the drums-only-GP case).

🤖 Generated with Claude Code

https://claude.ai/code/session_017xGPjDBF8NTwTK7VQvizix

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⚙️ Run configuration

Configuration used: defaults

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Run ID: 7736c822-5589-47d5-9798-33520ceba909

📥 Commits

Reviewing files that changed from the base of the PR and between 2d8ea40 and 85e3a58.

📒 Files selected for processing (7)

• CHANGELOG.md
• src/parts-view.js
• src/region.js
• src/state.js
• src/track-session.js
• tests/region_layout.test.mjs
• tests/selected_region_reset.test.mjs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch track-regions-p2

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