Section.Parse accepts numbers and comma lists as section types ([Verse, Chorus] parses as Chorus, [42] as undefined 42) #318

What's wrong

Section.TryParse (Semantics.Music/Section.cs ~L129) reads the header with Enum.TryParse<SectionType>(header[1..close], …). That call is too permissive for this header:

• It accepts any integer string and returns an undefined enum value.
• It accepts comma-separated names and ORs their values together, flags-style.
• It trims surrounding whitespace.

SectionType is a plain enum, so the first two cases produce wrong or undefined types.

Reproduction

Section.TryParse("[Verse, Chorus]\n4/4 C", out s)  -> true, s.Type == Chorus   (Verse=1 | Chorus=3 == 3)
Section.TryParse("[42]\n4/4 C", out s)             -> true, s.Type == (SectionType)42

Arrangement.Parse goes through the same path. An undefined type then flows into IsSameStructure and Form.

Suggested fix / acceptance criteria

• Accept only a single, defined member name. Two ways to do that:
  • require letters only, then call Enum.TryParse and Enum.IsDefined
  • look the name up in Enum.GetNames
• Keep matching case-sensitive as it is today, since ToString emits the exact member name.
• Tests: [Verse, Chorus], [42], [ Verse ] and [-1] are all rejected.