[patch] Spell fully and half-diminished sevenths as vii°7 and viiø7

[matt-edmondson]

Fixes #289

What was wrong

Key.QualitySuffix had two problems:

• It dropped the 7 from a fully diminished seventh, so Bdim7 became vii°, the same as the triad.
• It spelled a half-diminished seventh (m7b5) as vii°7, which ChordFromRomanNumeral reads back as Bdim7. The chord changed identity on a round trip.

Change (Semantics.Music/Key.cs)

• QualitySuffix:
  • A Diminished-quality chord with a Dominant (minor) seventh is now half-diminished: ø + 7.
  • A Diminished seventh always emits 7, so the fully diminished seventh becomes °7.
  • Triads (vii°), °maj7, and every non-diminished quality come out as before.
• ChordFromRomanNumeral: a leading ø in the suffix now passes through to Chord.Parse, which already reads ø/ø7 as m7b5. °, dim, + and aug are unchanged.
• Doc comment now lists viiø7 as an example.

Tests (RomanNumeralParseTests)

• Parse_IsInverseOfRomanNumeralOf now also round-trips vii°7 and viiø7.
• New tests:
  • Bdim gives vii° and Bdim7 gives vii°7.
  • Bm7b5 gives viiø7 and round-trips back to Bm7b5.
  • viiø7 parses to Bm7b5.
• The existing Parse_LeadingToneDiminishedSeventh (vii°7 → dim7) still passes, unchanged.

With Key.cs reverted to main, 4 of these fail. With the fix, the full Semantics.Test suite passes: 1469 passed, 0 failed, 8 skipped.

The triage suggested pairing this with #281/#284 (Chord.Parse reading °7/b5/Maj7). This PR doesn't change Chord.Parse. It only makes the Roman-numeral path use the °/ø vocabulary Chord.Parse already accepts.

🤖 Generated with Claude Code

https://claude.ai/code/session_01FRrKUmpjRrYrUchAi6wGqR

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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