Emit stable signatures for generic C# method display names

[Widthdom]

Summary

Generic C# method families fall back to display names like Name@line, omitting generic arity and parameter types; moving the method can therefore change its displayed identity.

Audit IDs: G-07. Reproduced with locally built cdidx v1.40.3 at 404db09489c305f17d94f37026e00564d3ac527a.

Existing issue relationship

Closed #3924 is adjacent generic overload/signature work.

Expected behavior and PR scope

Derive stable readable generic signatures from normalized arity and parameter types while preserving canonical identity and old-index compatibility. Keep this to display/disambiguation.

Implementation guide for Codex 5.6 sol xhigh

• Separate canonical identity, exact query aliases, and human display name.
• Normalize generic arity, type parameters/constraints policy, parameter modifiers/types, and nested generic types.
• Cover generic/non-generic overloads, extension/local/explicit-interface methods, tuples/nullable/arrays, and incomplete parse fallback.
• Avoid persisting source line as identity; document any additive metadata/fallback for old DBs.
• Add focused regression/contract tests as appropriate, run the full net8.0 and net9.0 suite, and add the required bilingual changelog.d/unreleased/ fragment.

Acceptance criteria

• Generic overloads have distinct useful signatures.
• Line movement does not rename the displayed method.
• Non-generic display and exact navigation remain stable.