Twelve-bar-blues Form does not round-trip: Form.Parse(form.ToString()) != form #447

What's wrong

In Semantics.Music/Form.cs:

• Form.Of(...) (about lines 85-86) sets Name = TwelveBarBlues based on the bar structure of the sections.
• TryParse (about line 117) derives the name from the letter pattern alone.
• ToString() (about line 123) returns only Pattern.
• Equals / GetHashCode (about lines 37-41) include Name.

The name is therefore lost when the form is written out and differs when it is parsed back.

Reproduction

Take a one-section 12-bar blues arrangement: C7×4 | F7×2 | C7×2 | G7 | F7 | C7×2.

• Form.Of(...) gives Pattern = "A", Name = TwelveBarBlues.
• form.ToString() gives "A".
• Form.Parse("A") gives Name = ThroughComposed.
• Form.Parse(form.ToString()) == form is false.

Why it matters

The music type-safe-factories work promises a round trip for every type:

• plan Task 16 requires Parse(x.ToString()) == x.
• The design spec`` states that Form "already round-trips".

Anything that persists a Form as text and compares it later breaks for blues forms, for example caching or deduplicating analyses.

Suggested fix (needs a decision)

Pick one:

• (a) Treat Name as derived. Exclude it from Equals/GetHashCode, and document that Parse names forms from the pattern only.
• (b) Make the canonical text carry the name, for example A (TwelveBarBlues), and parse it back.

Acceptance criteria

• A round-trip test for a twelve-bar-blues Form passes.
• The chosen semantics are documented on Form.

Related: #384 (AABA naming should depend on section length) causes the same symptom for AABA forms, but it is a separate fix.