Chord.Parse stacks the natural 9th/11th under an altered one: "C13#11" sounds both F and F#, "C13b9" both Db and D

[matt-edmondson]

What's wrong

In Chord.ApplyExtensions (Semantics.Music/Chord.cs:437-444), the 13 branch always adds Nine | Eleven | Thirteen and the 11 branch always adds Nine | Eleven. Only the bare 9 branch (:445-451) skips Nine when FlatNine or SharpNine is already set. So an altered tension is added next to its natural version instead of replacing it.

Failure scenario

Probe output against HEAD:

Input	Tensions	Tones	Problem
C13b9	FlatNine, Nine, Eleven, Thirteen	[0,4,7,10,13,14,17,21]	Db and D
C13#11	…	[…,17,18,…]	F and F#
C13#9	…	[…,14,15,…]	D and D#
C11b9	…	[…,13,14,…]	Db and D

Anything built on ChordTones inherits the extra notes, Voice() included, so a voicing plays a minor second cluster the symbol never asked for.

The round-trip corpus doesn't catch this because the wrong chord round-trips: C13b9 prints C713b9, which reparses to the same wrong set.

Suggested fix

Apply the rule the 9 branch already uses:

• In the 13 and 11 branches, add Nine only when neither FlatNine nor SharpNine is set.
• Add Eleven only when SharpEleven is not set.

Acceptance criteria

• Tone assertions for C13b9, C13#9, C13#11, C11b9 and a combined one such as C13b9#11 contain the altered tension and not the natural one.
• Existing C13, C11 and C9 tone assertions are unchanged.

[matt-edmondson]

Triage

• Category: Bug
• Priority: Medium. It produces wrong chord tones, with a minor-second cluster, for common altered-dominant symbols. The fix is small and local.
• Area / suggested assignment: Semantics.Music, Chord.ApplyExtensions.
• Related: Chord.Parse reads "Cm7#5", "Cm#5" and "Cm+" as augmented chords with a major third (Cm7#5 comes back as Caug7) #340 is in the same parser (DetermineQuality), and Chord.ToString() writes an altered tension straight after the root: "C(#11)" prints as "C#11" and reparses as a C# dominant eleventh #336 covers ToString of altered tensions. Consider one Chord-parser PR for Chord.ToString() writes an altered tension straight after the root: "C(#11)" prints as "C#11" and reparses as a C# dominant eleventh #336, Chord.Parse reads "Cm7#5", "Cm#5" and "Cm+" as augmented chords with a major third (Cm7#5 comes back as Caug7) #340 and Chord.Parse stacks the natural 9th/11th under an altered one: "C13#11" sounds both F and F#, "C13b9" both Db and D #341. Not a duplicate.
• In progress? No open PR matches. Open Semantics PRs Convert temperature differences with the factor alone, without the scale offset [patch] #342 and [minor] Keep Vector0 magnitudes non-negative through unary minus and scalar * and / #343 cover temperature and vectors.
• Notes: Add a tone-set assertion to the round-trip corpus. As the issue notes, the round-trip check can't catch wrong chords that reparse consistently.

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[matt-edmondson]

Covered by #360, which redesigns this bug cluster as a whole. Implement it through that issue rather than individually.

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