Preserve C# explicit-interface symbol identities

[Widthdom]

Summary

• Preserve C# explicit-interface qualifiers and method generic arity in the persisted symbol identity while keeping short display aliases discoverable.
• Apply the identity consistently across extraction, SQLite schema/migrations, fold backfill/readiness, exact search/count/definition/inspect, diff, outline, LSP, and impact traversal.
• Keep qualified impact traversal scoped to every matched interface declaration/explicit implementation ID, excluding same-named public members.
• Preserve the explicit qualifier when a supported post-extraction hook performs a name-only rename.

Validation

• dotnet build CodeIndex.sln -c Debug --no-restore
• dotnet build CodeIndex.sln -c Release --no-restore
• Issue Preserve explicit-interface qualifiers in C# symbol identity #4866 tests: 14/14 passed in Debug and Release on both .NET 8 and .NET 9
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• dotnet run --project tools/CodeIndex.Changelog -- check (54 fragments validated)
• git diff --check origin/main...HEAD
• Full cdidx index . --rebuild completed for 1,280 files with zero warnings/errors; cdidx status --check reports the index, HEAD, workspace, reference graph, folds, and C# symbol-name contract as current
• Two Codex adversarial review rounds completed; all actionable findings were addressed

Documentation and changelog

• Updated the English and Japanese sections in README.md, DEVELOPER_GUIDE.md, and TESTING_GUIDE.md.
• Added changelog.d/unreleased/4866.fixed.md.

Follow-up candidates

• None.

Fixes #4866