fix(theory): render half-diminished colour romans as viiø7 (refs #52)

[wiradifit]

Summary

Completes the remaining acceptance criterion of #52.

The out-of-range crash itself (unconditional pcs[5] indexing in the tension layer) is already resolved on main — that layer now looks sectors up by heptatonic degree with optional bindings, and I confirmed there are no unguarded pcs[N] subscripts left in HarmonicWheel.swift. This PR fixes the issue's second checklist item:

Colour-layer roman numeral for vii renders vii°7 — should be viiø7

WheelDegreeBuilder.colourQuality correctly maps the diminished degree to .halfDiminished7, but the roman-numeral builder appended a plain "7" to every non-.major7 quality, producing vii°7 — the glyph for a fully diminished seventh.

Changes

• Sources/XPadTheory/HarmonicWheel.swift — the colour-layer roman closure now switches on chord quality:
  • .major7 → <roman>maj7 (unchanged)
  • .halfDiminished7 → strips the trailing ° from the degree roman and appends ø7 → viiø7
  • everything else → <roman>7 (unchanged)
• Tests/XPadTheoryTests/HarmonicWheelPentatonicTests.swift (new) — regression coverage for both halves of HarmonicWheel tension layer indexes pcs[5] unconditionally — crash on pentatonic scales #52:
  • pentatonicMajor, pentatonicMinor, and blues construct through ordinary public API without trapping
  • heptatonic scales still construct
  • the C-major colour layer contains a half-diminished vii whose roman numeral equals viiø7
  • dominant-seventh romans remain V7

Verification

Focused suite executed on Linux (Swift 6.2 toolchain, pure-Swift theory modules only):

Test Suite 'HarmonicWheelPentatonicTests' passed
   Executed 4 tests, with 0 failures (0 unexpected)

The UI/MIDI/audio modules require Apple frameworks, so the full-package swift test run belongs to this repo's macOS CI workflow per CONTRIBUTING.md.

Refs #52

Summary by Sourcery

Correct half-diminished colour roman-numeral notation and protect harmonic-wheel construction and rendering with regression tests.

Bug Fixes:

• Render half-diminished colour-layer vii chords as viiø7 instead of the fully diminished vii°7 notation.

Tests:

• Add regression coverage for constructing pentatonic, blues, and heptatonic harmonic wheels, along with half-diminished and dominant-seventh roman numeral rendering.

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[sourcery-ai]

Reviewer's Guide

The PR updates colour-layer roman numeral generation to render half-diminished vii as viiø7 while preserving existing major-seventh and dominant-seventh output, and adds regression coverage for the glyph fix and construction of wheels from scales with fewer than seven pitch classes.

Sequence diagram for half-diminished vii roman generation

sequenceDiagram
    participant Wheel as HarmonicWheel
    participant Builder as WheelDegreeBuilder
    participant Roman as Colour roman closure
    Wheel->>Roman: build(spec)
    Roman->>Builder: colourQuality(for: spec, isMinor: isMinor)
    Builder-->>Roman: halfDiminished7
    Roman->>Roman: String(spec.roman.dropLast())
    Roman-->>Wheel: viiø7

Loading

Flow diagram for colour-layer roman numeral rendering

flowchart TD
    S[Colour layer spec] --> Q[WheelDegreeBuilder.colourQuality]
    Q --> M{Chord quality}
    M -->|major7| A[Return roman + maj7]
    M -->|halfDiminished7| B[Remove trailing degree symbol]
    B --> C[Return base + ø7]
    M -->|other| D[Return roman + 7]

Loading

File-Level Changes

Change	Details	Files
Corrected colour-layer roman numeral rendering for half-diminished seventh chords.	Added quality-specific rendering for major seventh, half-diminished seventh, and other seventh chords. Replaced the trailing fully diminished glyph with ø before appending 7 for half-diminished chords.	Sources/XPadTheory/HarmonicWheel.swift
Added regression tests covering sub-heptatonic construction and seventh-chord roman numeral output.	Verified pentatonic and blues wheels construct without trapping, alongside heptatonic wheels. Asserted C-major colour vii renders as viiø7. Asserted dominant seventh remains V7.	Tests/XPadTheoryTests/HarmonicWheelPentatonicTests.swift

Possibly linked issues

• HarmonicWheel tension layer indexes pcs[5] unconditionally — crash on pentatonic scales #52: PR addresses issue HarmonicWheel tension layer indexes pcs[5] unconditionally — crash on pentatonic scales #52 by rendering viiø7 correctly and testing pentatonic construction without crashes.

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Sourcery assessment

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