feat(editor): drum grid row density — Full / Compact family rows

[ChrisBeWithYou]

P2 of the view-modality train (EDITOR-VIEW-MODALITY-DESIGN.md VD.1, decision V6: one evolving drum grid with density presets — never a parallel "game-style" editor, which could only lose information). Independent of the P1 stack; branched off current main.

What

• Drum lane geometry routes through a density lane table: Full = one row per piece (today''s grid, byte-for-byte), Compact = the community 7-row family shape — crash / hi-hat / ride / toms / floor toms / snare / kick, mirroring core lib/drums.py PRESET_RB4 family boundaries — so the backbeat reads at a glance.
• Strictly render/selection grouping — the V6 guarantee: hits keep their real piece-ids and per-piece colors (a compact row shows members distinctly), every piece lives on exactly one row (collapsing can never hide data), and hit lookup matches any row member.
• Authoring semantics in Compact, documented in the button title: adding writes the family''s canonical piece (crash→crash_l, hi-hat→hh_closed, ride→ride…); a time-only drag keeps the hit''s original piece (hh_open never silently becomes hh_closed); crossing rows assigns the target family''s canonical. In Full, both rules reduce to exactly today''s behavior.
• "Rows: Full/Compact" button next to Edit Drums (visible in drum mode only, per-draw memoized), registry command toggleDrumDensity, localStorage pref — never pack data.

Tests

tests/drum_density.test.js (7): full-mode identity with the physical-kit order, total coverage + uniqueness of the compact mapping (no piece falls off the grid), canonical row-membership, junk-pref fallback to Full, table purity (no config mutation leaks), and the same-row/cross-row drag semantics. Full suite green except the pre-existing CRLF section_coverage failure (#116 fixes it).

Fresh region vs the open queue — drum lane geometry + one toolbar button. (Registry/draw-wrapper insertion points may keep-both against sibling PRs, the usual resolution.)

🤖 Generated with Claude Code

https://claude.ai/code/session_01JEoFeTPSnz4NpwwCG52hnu

Summary by CodeRabbit

• New Features

  • Added a drum grid density toggle with Full and Compact views.
  • Compact mode groups drum pieces into shared family rows while keeping piece selection and colors consistent.
  • Dragging between rows now moves hits to the destination row’s mapped piece, and the density button updates to show the current mode.

• Bug Fixes

  • Switching grid density now clears the current selection and keeps drum lane interactions aligned with the displayed rows.

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📝 Walkthrough

Walkthrough

Introduces a drum grid row density toggle (Full/Compact) with a family-lane mapping table, pure helper functions, localStorage persistence, updated rendering/hit-testing/drag logic keyed off lane index, a new UI toggle button, a Node test suite, and a changelog entry.

Changes

Drum row density feature

Layer / File(s)	Summary
Density mode and lane table core screen.js	Adds DRUM_COMPACT_LANES, pure lane-table/lane-index helpers, localStorage-backed density preference caching, _editorToggleDrumDensity(), and wires the toggleDrumDensity command into the shortcut list and command router.
Rendering and hit-testing via lane table screen.js	Reworks _drumPieceCount, _drumPieceAtY, _drumHitAtPoint, lane-grid drawing, lane label rendering, and dot/hit positioning to use the density lane table instead of DRUM_PIECE_ORDER indexing.
Drag remapping and density toggle button screen.js	Updates drag lane-move remap logic to clamp and remap pieces via lane canonicals, updates per-hit lane placement, and adds/refreshes the editor-drum-density-btn UI control in the draw() lifecycle.
Tests and changelog tests/drum_density.test.js, CHANGELOG.md	Adds a test suite validating full/compact lane shapes, family canonicals, unknown-density fallback, lane lookups, projection purity, and drag remap semantics, plus a changelog entry describing the feature.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Sequence Diagram(s)

sequenceDiagram
  participant User
  participant UIButton as editor-drum-density-btn
  participant Toggle as _editorToggleDrumDensity
  participant LaneTable as _drumLanes / lane helpers
  participant Renderer as Drum grid renderer

  User->>UIButton: click toggle
  UIButton->>Toggle: onclick()
  Toggle->>Toggle: flip density, persist preference
  Toggle->>Toggle: clear drum selection
  Toggle->>Renderer: redraw()
  Renderer->>LaneTable: _drumLanes() / _drumLaneIdxForPiece()
  LaneTable-->>Renderer: lane rows, canonical pieces
  Renderer-->>User: updated grid + status text

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Related Issues: None referenced.

Related PRs: None referenced.

Suggested labels: enhancement, editor, drum-grid

Suggested reviewers: None identified from the provided context.

🐰 A drummer's rows once stood alone,
now families gather, each finds a home,
compact or full, the beat stays true,
seven lanes wide, or one-for-you,
a button clicks—the grid renews.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely summarizes the main editor change: adding full/compact drum grid row density modes.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

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• Create PR with unit tests
• Commit unit tests in branch feat/editor-drum-density

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