feat(editor): Tracks-view regions become glass banners with the notes tucked below the title

[ChrisBeWithYou]

What & why

Christian asked to rework how the Tracks-view region containers look — the old
hairline+spine blocks blended into the lane and read weakly. After a couple of
render passes (A–D, then an A×C hybrid set), the pick was a glass banner:
keep the airy top-lit glass fill so the note silhouette stays the dominant read,
but give each region a proper titled band — and crucially, inset the notes
below the band so they no longer run up under the header line.

What changed

Render-only change to the Tracks (Parts) overview:

• Each region draws a translucent kind-colour title band (blue master / teal
  stem / green guitar / orange bass / purple drums / ice keys) over a top-lit
  glass fill that fades to almost nothing, with a rounded border that accents
  and glows when selected.
• The note silhouette / audio waveform is inset below the band, so notes
  never cross the header line.
• The region name is vertically centred in the band so descenders (p, y, g)
  stay inside it.
• Short lanes (< ~23px) degrade to a bare colour spine — the band never
  crushes the content.

Geometry, hit-testing, bar-snap drag and the drop ghost are untouched. The one
new bit of logic — the banner-height clamp _regionBannerH — lives with the
other region-layout pures in src/region.js and is pinned by a unit test.

Testing

• npm test — 304 pass (the 2 pre-existing failures, song_fit and
  mixer_meter_teardown, fail identically on main; unrelated to this change).
• npm run lint — 0 errors.
• New: tests/region_banner.test.mjs (banner-height clamp / short-lane guard;
  fails on main where _regionBannerH doesn't exist).
• Runtime-verified by serving this branch through the real host and opening
  the Tracks view on a real pack (AC/DC – Back In Black): banners, centred names,
  notes-below-the-header, kind colours, selection glow, and bounded regions all
  render correctly.

🤖 Generated with Claude Code

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Review details

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: bfcc34b9-4375-4c80-abbb-808b51e197a0

📥 Commits

Reviewing files that changed from the base of the PR and between 021730a and cc8df38.

📒 Files selected for processing (4)

• CHANGELOG.md
• src/parts-view.js
• src/region.js
• tests/region_banner.test.mjs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch region-glass-banner

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