feat(music): type-safe factories and canonical round-trip ToString/Parse

[matt-edmondson]

Summary

Gives Semantics.Music compiler-enforced construction and round-trippable text. Implements the docs/superpowers/plans/2026-07-02-music-type-safe-factories.md plan (20 tasks, TDD, one commit each).

• New NoteLetter / Accidental value-backed enums (underlying value = natural pitch class / semitone offset) and typed factories: PitchClass.Create(NoteLetter, Accidental), Pitch.Create(NoteLetter, Accidental, octave).
• Canonical ToString() + Parse/TryParse on every in-scope type (PitchClass, Pitch, Interval, Duration, TimeSignature, Velocity, Tempo, Scale, Key, Chord, ChordEvent, Note, Rest, Form, Progression, Section, Arrangement), with a tested Parse(x.ToString()) == x round-trip. Parse throws FormatException; TryParse(string?, out T?) never throws.
• Renames to the BCL convention: Pitch.FromName/Mode.FromName → Parse/TryParse; Form.FromPattern → Parse/TryParse.
• Shared internal Notation scanner dedupes the note-letter/accidental parsing across Pitch, Chord, and roman-numeral parsing.

Breaking changes

• Progression.Parse(string) format changed. The bar-delimited lead-sheet parser ("C | Am | F | G") is replaced by the chart-style format ("4/4 Dm7 / G7 / | Cmaj7 / / /"): a leading time signature is now required, and beat slashes carry duration. All in-repo call sites migrated. (Decision made during implementation; the plan had not accounted for the pre-existing analysis-layer parser.)
• Structural value equality added to Form, Progression, and Arrangement (Section gets it transitively) so Parse(x.ToString()) == x holds — these records previously compared their list members by reference.

Suggest tagging the merge commit [major] (or [minor] if the changed Progression.Parse from the recent analysis layer is considered not-yet-stable API).

Test plan

• 1037/1037 tests pass; clean multi-target build (net8.0–net10.0 + netstandard2.0/netstandard2.1), 0 warnings.
• New round-trip suites for the chord corpus and the three chart-style aggregates; new/merged tests for every type.

🤖 Generated with Claude Code

[sonarqubecloud]

Quality Gate failed

Failed conditions
2 Security Hotspots
C Security Rating on New Code (required ≥ A)

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