NamedForm.VerseChorus is declared but Form never produces it; the spec's own verse–chorus example comes out Unknown #321

Plan reference

• Enum: https://github.com/ktsu-dev/Semantics/blob/7852eab73d673ab42ff1ad10cb1ad1124b8b0122/docs/superpowers/specs/2026-07-01-music-analysis-aggregate-design.md#L238-L241
• Example song map: https://github.com/ktsu-dev/Semantics/blob/7852eab73d673ab42ff1ad10cb1ad1124b8b0122/docs/superpowers/specs/2026-07-01-music-analysis-aggregate-design.md#L230-L233

enum NamedForm { ThirtyTwoBarAABA, TwelveBarBlues, VerseChorus, Binary, Ternary, Rondo, Strophic, ThroughComposed, Unknown }

Repetition and song-map semantics are expressed by repeating Section entries in order (e.g. Verse, Chorus, Verse, Chorus, Bridge, Chorus).

What exists today

• Semantics.Music/NamedForm.cs declares VerseChorus ("Verse-chorus form").
• Form.RecognizePattern (Semantics.Music/Form.cs ~L103-112) has no branch that returns it, and no test references it.
• Neither the spec nor the code records a deliberate deviation. Compare ChromaticKind.AugmentedSixth, whose doc explicitly marks it as reserved.

What's missing

• There is no recognition rule, so the spec's own example (Verse, Chorus, Verse, Chorus, Bridge, Chorus → ABABCB) comes out Unknown.
• The spec gives no letter pattern for this form. Recognition naturally keys on SectionType, which Form.Of has and Form.Parse(pattern) does not.

Acceptance criteria

• Form.Of returns VerseChorus when an arrangement alternates Verse and Chorus sections: at least two of each, with each verse followed by a chorus. Intro, PreChorus, Bridge and Outro sections around them are allowed. Pin down the exact rule in the XML doc.
• Form.Parse keeps letter-only recognition, and its doc states that it cannot produce VerseChorus (as with TwelveBarBlues).
• Tests:
  • The spec example yields VerseChorus.
  • An all-verse arrangement stays Strophic.
• If the form is not wanted after all, mark the member as reserved in its XML doc instead, as AugmentedSixth is.

Dependencies

None. #314 touches the same method but does not overlap with this.