Structure tags with declared Members no longer register the bare parent
name in `_tagsByName` — reading it would return Good/null, which is
misleading. Clients read individual member paths. Both the member
fan-out and the scalar-tag paths now perform a duplicate-key check that
throws `InvalidOperationException` naming both colliding entries (fail-
fast, consistent with the AbCipHostAddress validation pattern).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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