feat(editor): chord-at-cursor readout at the playhead

[ChrisBeWithYou]

DAW roadmap 4.17 — the chord-at-cursor third of the item. (Passive key hints + fill-blanks deferred to a follow-up so this stays atomic.) Editor-only.

What

A small, read-only chord name appears next to the measure display, naming whatever sounds at the playhead — C, Am, Gmaj7, D#6, Csus4, E5, …

• Exact-match vocabulary (@pure:chord-id): common triads, sevenths, sus, and the power-chord dyad, matched exactly against the sounding pitch-class set — so a name shows only when it's certain. — when notes sound but form no named chord; blank when nothing sounds.
• Bass breaks the genuine ties. m7 and 6 are the same four pitch classes (a Cm7 voiced over E♭ is D#6); augmented and dim7 are symmetric. The lowest sounding pitch class picks the spelling. (Sharp spelling throughout — enharmonic-correct labels are the separate 4.16a item.)
• Pitch-correct per part. Fretted parts resolve to sounding pitch (capo- and tuning-aware, via _rollMidiForNote + _rollPitchCtx); keys parts use their packed pitch; octave doublings collapse. _notesSoundingAtPure gathers notes ringing at the time (onset-inclusive, sustain-aware, zero-sustain notes linger a min-dur so a struck chord registers).
• Wired into updateTimeDisplay; new #editor-chord-display span. Drum/tempo modes show nothing. Never edits.

Tests — tests/chord_at_cursor.test.js (11, all fail on main)

pc-set dedupe/rounding; triads, sevenths, sus, dim, aug, power chords; the m7-vs-6 and sus2-vs-sus4 bass disambiguation; unmatched clusters → null; the sounding-window predicate (onset-inclusive, sustain, gaps, NaN, non-array); and an end-to-end 6-string E-major voicing naming as E. Full JS suite green.

Scope / fresh region

New @pure:chord-id block after @pure:measure-readout, a updateChordDisplay beside updateMeasureDisplay, one call in updateTimeDisplay, and one HTML span. No core, no spec; no overlap with the open PRs (#112/#125/#126/#127/#128/#129).

Summary by CodeRabbit

• New Features

  • Added a read-only “chord at the playhead” label in the editor toolbar, shown beside the time/measure display.
  • The label updates live with playback, using exact chord vocabulary matching and a clear output behavior for sounding vs silent/unclear cases.

• Tests

  • Added tests validating chord identification, sounding-note timing rules, memoization/cache hit behavior, and an end-to-end chord readout scenario.

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Review details

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 9fef58b6-f5d5-462f-b035-344dfda40723

📥 Commits

Reviewing files that changed from the base of the PR and between c68018c and 836c035.

📒 Files selected for processing (3)

• CHANGELOG.md
• screen.html
• screen.js

📝 Walkthrough

Walkthrough

Adds a read-only chord-at-cursor readout with a new toolbar display, pure chord-identification logic in screen.js, integration into the time-display refresh path, tests for the pure helpers, and a changelog entry.

Changes

Chord-at-cursor readout

Layer / File(s)	Summary
Pure chord identification helpers screen.js	Adds CHORD_FORMULAS and pure helpers to derive pitch-class sets, identify chords with bass tie-breaking, determine sounding notes at a time, and validate cached readout intervals.
UI wiring and display element screen.html, screen.js	Adds the editor-chord-display placeholder, updates it from the time display refresh path, and renders the chord readout text and title.
Tests and changelog tests/chord_at_cursor.test.js, CHANGELOG.md	Adds a Node test harness for the pure chord helpers, covers chord naming and sounding-window behavior, and documents the feature in the changelog.

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

🚥 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 describes the main change: an editor chord readout at the playhead.
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

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/editor-chord-at-cursor

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