TryParse now rejects three classes of malformed PCCC address:
- Sub-element + bit-index together (e.g. T4:0.ACC/2) — never valid in PCCC
- File number on I/O/S system files (e.g. I3:0, S2:1) — single-letter only
- Sub-element on non-T/C/R files (e.g. B3:0.DN, N7:0.FOO) — only Timer,
Counter, and Control files carry structured elements
New helper predicates IsNoFileNumberLetter / IsSubElementFileLetter
keep the parser's intent clear. Regression tests added in AbLegacyAddressTests.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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