Preserve explicit-interface qualifiers in C# symbol identity

[Widthdom]

Summary

Explicit-interface members such as IFoo.Changed and IFoo.Run<T> collapse into ordinary Changed/Run families, losing the qualifier that distinguishes implementations.

Audit IDs: G-06. Reproduced with locally built cdidx v1.40.3 at 404db09489c305f17d94f37026e00564d3ac527a.

Existing issue relationship

Closed #333, #351, and #356 are adjacent explicit-interface/identity work; no exact open issue exists.

Expected behavior and PR scope

Canonical identity and exact navigation distinguish explicit-interface members from public/other implementations while keeping readable display output. Keep this PR to C# symbol identity, extraction, and resolution for explicit members.

Implementation guide for Codex 5.6 sol xhigh

• Retain interface qualifier, member kind, generic arity, and normalized signature in canonical identity.
• Define display name versus exact query name explicitly so compatibility aliases do not merge families.
• Cover properties/events/indexers, generic/non-generic methods, multiple interfaces, inherited interfaces, and same-named public members.
• Verify symbols, definition, references, inspect, outline, and LSP navigation.
• Add focused regression/contract tests, run the full net8.0 and net9.0 suite, and add the required bilingual changelog.d/unreleased/ fragment.

Acceptance criteria

• Exact queries can distinguish IFoo.Run<T> from ordinary Run.
• Explicit properties/events do not merge with unrelated families.
• Existing unqualified discovery remains usable through documented ranking/aliases.

[Widthdom]

Work started on branch fix-issue4866.