Refuse a form with more distinct sections than letters [patch]

[matt-edmondson]

Fixes #314

What changed

Form.Of gave each structurally distinct section the letter 'A' + index with no upper bound. The 27th distinct section was labelled '[', which Form.Parse rejects, so Parse(ToString()) failed on a Form the library had produced.

This takes the fail-fast option from the issue, which triage named as the smaller change:

• Form.Of throws ArgumentException when an arrangement has more than 26 structurally distinct sections.
• The limit is exposed as Form.MaxDistinctSections and documented on Form.Of and Arrangement.Form.
• Repeats of earlier sections still label normally, so a 27-section arrangement with only 26 distinct structures works.

Tests

FormTests gains three cases:

• 26 distinct sections give A–Z, and the result round-trips through Parse.
• 27 distinct sections throw. This test fails on main with "no exception was thrown".
• 26 distinct sections plus one repeat give A–Z followed by A.

All 354 Music tests pass. The local Sonar build reports nothing in the changed files.

🤖 Generated with Claude Code

https://claude.ai/code/session_01SWJoWcEC3BuVS7pSPco7mA

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Generated by Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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