Section.ToString() doesn't round-trip labels containing "(" or a quote: "Verse (reprise)" fails to parse and a quoted word truncates the label #312

What's wrong

Section.ToString() writes the label raw between double quotes (Semantics.Music/Section.cs:79: sb.Append(" \"").Append(Label).Append('"')). Section.TryParse reads the header back with plain IndexOf:

• The label ends at the first " after the opening quote (Section.cs:135-144).
• The key starts at the first ( anywhere after ] (Section.cs:148). That includes a ( inside the label.

Label is documented as a free-form human label, and ToString as the canonical section text. The existing round-trip tests for Section and Arrangement don't use such labels.

Reproduction (run against the built assembly)

var prog = Progression.Parse("4/4 C F G C");
Section.TryParse(Section.Create(SectionType.Verse, prog, "Verse (reprise)").ToString(), out _)
    // false: "(reprise)" is handed to Key.TryParse
Section.TryParse(Section.Create(SectionType.Verse, prog, "Outro (fade)", Key.Parse("C major")).ToString(), out _)
    // false
Section.TryParse(Section.Create(SectionType.Verse, prog, "The \"big\" one").ToString(), out var r)
    // true, but r.Label == "The " (silently truncated)
Arrangement.TryParse(Arrangement.Create(Key.Parse("C major"), [sectionWithParenLabel]).ToString(), out _)
    // false: one such section makes the whole arrangement unparseable

A label containing a newline also splits the header line.

Why it matters

Labels such as "Chorus (x2)", "Verse (reprise)" and "Outro (fade)" are ordinary chart annotations. Today a section with one of them serializes to text the library can't read back, and a quote in a label is silently lost. This breaks the Parse(x.ToString()) == x contract that the type-safe-factories plan set for every music type.

Suggested fix / acceptance criteria

• Pick one of these:
  • Escape " and \ inside the label on output, reject or escape newlines, and parse the quoted label with an escape-aware scanner.
  • Reject such labels in Section.Create.
• Either way, search for the key's ( only after the label's closing quote.
• Tests: Section.Parse(s.ToString()) == s for labels containing (, ), " and ], with and without a local key, and the same round trip through Arrangement.