Chord.Parse rejects Cmaj7sus4 / Cmaj9sus4 / Cmaj7sus2, yet Chord.ToString() prints exactly those symbols, so a Progression holding one cannot be re-parsed #368

What's wrong

A suspended chord with a major seventh is a valid Chord: new Chord { Root = C, Quality = Sus4, Seventh = Major } has tones [0,5,7,11]. ToString() prints it as Cmaj7sus4, but Chord.Parse rejects that text and every other spelling of the chord. These inputs all make TryParse return false: Cmaj7sus4, CM7sus4, Cmaj7sus, Cmaj9sus4, Cmaj7sus2 and Cmaj7(sus4). C7sus4 is accepted, so only the major-seventh case is missing.

Confirmed by compiling Semantics.Music at 8d68f2a into a scratch console app:

Sus4+Major+None: ToString=Cmaj7sus4 tones=[0,5,7,11] TryParse=False
Sus4+Major+Nine: ToString=Cmaj9sus4 tones=[0,5,7,11,14] TryParse=False
Sus2+Major+None: ToString=Cmaj7sus2 tones=[0,2,7,11] TryParse=False
Progression.Parse("4/4  Cmaj7sus4 / / /") -> FormatException

The roman-numeral path is broken the same way. In C major, Key.RomanNumeralOf(Cmaj7sus4) returns "Isus4maj7", and Key.ChordFromRomanNumeral("Isus4maj7") throws FormatException: Invalid chord symbol 'Csus4maj7'.

Why it matters

• Cmaj7sus4 is an ordinary symbol in jazz and pop charts, and a chart containing it cannot be loaded.
• A Progression, Section or Arrangement built in code with this chord produces ToString() text that its own Parse rejects, which breaks the advertised round-trip contract.
• The implementation is stricter than its own documented grammar. Semantics.Music/README.md (lines 90–98) gives body = [ quality ] [ extension ] [ suspension ] with extension = [ majorWord ] number, which accepts maj7 followed by sus4. Redesign chord symbols: one grammar for Chord.Parse and Chord.ToString (fixes #300 #322 #336 #340 #341 #344 #345 #346 #358) #360 rejected sus only when combined with a quality (m/dim/aug), and maj belongs to the extension, not the quality.

Where

• Semantics.Music/ChordSymbolReader.cs:601-602: HasInvalidSuspension() includes MajorMarker, so it rejects maj7sus4.
• Semantics.Music/ChordSymbolWriter.cs:127-130: HasSupportedStructure treats Sus2/Sus4 with SeventhType.Major as inexpressible. That is why EveryExpressibleChordRoundTrips (Semantics.Test/Music/ChordRoundTripTests.cs) skips this case instead of catching it.
• Semantics.Music/Key.cs:232-259: QualitySuffix writes sus4 before maj7 (Isus4maj7).

Suggested fix / acceptance criteria

• Drop MajorMarker from HasInvalidSuspension(), while still rejecting m/dim/aug/+ with sus.
• Remove the Sus + Major-seventh exclusion from ChordSymbolWriter.HasSupportedStructure, so the exhaustive round-trip test covers it.
• These parse to the right tones and round-trip through ToString():
  • Chord.Parse("Cmaj7sus4") → Sus4 + Major seventh, [0,5,7,11]
  • Cmaj9sus4 → [0,5,7,11,14]
  • CM7sus2 → [0,2,7,11]
• Key.QualitySuffix emits the seventh before the suspension (Imaj7sus4), so ChordFromRomanNumeral(RomanNumeralOf(c)) works for these chords.
• New cases are added to ChordGrammarTests (with tones asserted) and to the ChordRoundTripTests corpus.