Key.RomanNumeralOf drops the seventh from a fully diminished 7th ("Bdim7" → "vii°"), and a half-diminished 7th round-trips to fully diminished #289

What's wrong

QualitySuffix in Semantics.Music/Key.cs has these arms:

SeventhType.Diminished => chord.Quality == ChordQuality.Diminished ? "" : "7",
...
ChordQuality.Diminished => "°",

This produces two wrong results:

• A fully diminished seventh (Diminished quality with a Diminished seventh) gets "°" plus an empty suffix, so Bdim7 becomes vii°. That is the same numeral a plain Bdim triad gets, so the seventh is lost.
• A half-diminished seventh (m7b5, which is Diminished quality with a Dominant seventh) gets "°7". But ChordFromRomanNumeral("vii°7") reads "°" as "dim" and builds Bdim7, the fully diminished chord. RomanNumeralParseTests.Parse_LeadingToneDiminishedSeventh asserts that reading, so the round trip changes the chord.

Repro (C major)

Key c = Key.Create(PitchClass.Create(0), Mode.Major);
c.RomanNumeralOf(Chord.Parse("Bdim"));   // "vii°"
c.RomanNumeralOf(Chord.Parse("Bdim7"));  // "vii°"   expected "vii°7"
c.ChordFromRomanNumeral(c.RomanNumeralOf(Chord.Parse("Bm7b5")));
// "vii°7" -> Bdim7 (Seventh = Diminished, 9-semitone seventh); expected half-diminished (10 semitones)

This was reproduced with a temporary MSTest on net10.0 at 8c06aba.

Why it matters

The two sevenths on the leading tone are the most common place Roman-numeral analysis has to tell fully diminished and half-diminished apart: vii°7 in minor, viiø7 in major. Right now a triad and a diminished seventh get the same label. A half-diminished chord also changes identity when it goes through RomanNumeralOf and then ChordFromRomanNumeral.

Suggested fix / acceptance criteria

• QualitySuffix emits °7 for Diminished + Diminished seventh and ø7 for Diminished + Dominant seventh (half-diminished).
• ChordFromRomanNumeral reads ø (and ø7) as m7b5.
• vii°7 and viiø7 are added to Parse_IsInverseOfRomanNumeralOf, so both survive the round trip.

This is separate from #281, which is about Chord.Parse("C°7").