Chord.Parse cannot parse the "6/9" chord symbol (and any bare tension after a slash) #248

What's wrong

Chord.TryParse unconditionally treats everything after the first / as a slash-bass note: it calls TryParseRoot on the substring after /, which requires the first character to be a note letter A–G (Notation.TryReadNoteLetter). The extremely common jazz "six-nine" chord symbol — C6/9, G6/9, F6/9 — isn't a slash-bass chord at all; the /9 denotes an added ninth stacked on a sixth chord, not a bass override. Because "9" isn't a valid note letter, TryParseRoot fails, TryReadRoot returns false, and Chord.TryParse("C6/9") returns false / Chord.Parse("C6/9") throws FormatException.

Why it matters

The package's own documentation lists sixths, extensions (9/11/13), and slash bass as supported features, but Chord.Parse("C6/9") throws FormatException: Invalid chord symbol 'C6/9'. — a chord symbol found on virtually any jazz lead sheet.

Suggested fix / acceptance criteria

When the token after / fails to parse as a note letter, fall back to checking whether the whole head (e.g. "C6/9") matches the known six-nine idiom (bare 9 immediately after a 6), and encode it as Sixth = Natural, Tensions |= Nine with no bass, rather than failing outright. Add a round-trip test: Chord.Parse("C6/9").ChordTones() should equal Chord.Parse("Cadd9").ChordTones() plus the natural sixth, and Chord.Parse("C6/9").ToString() should re-parse to an equal chord.

File: Semantics.Music/Chord.cs (TryReadRoot, TryParseRoot)