Fix unresolved exact caller identity fallback

[Widthdom]

Summary

• Require resolved target identities before exact C# caller and impact queries treat reference rows as confirmed graph edges.
• Preserve unresolved same-name rows only for broad discovery, while keeping language-unspecified non-C# graph traversal and synchronizing confidence, degraded-state, and count-authority metadata across CLI, compact output, and MCP.
• Add regression coverage for missing definitions, same-leaf collisions, transitive unresolved edges, mixed-language callers, stale/incomplete graphs, caps, compact projections, inspect, hotspots, CLI, and MCP.

Validation

• dotnet build CodeIndex.sln -c Release (0 warnings, 0 errors)
• Full .NET 9 suite before adversarial-review follow-ups: 11,440 passed, 428 skipped, 0 failed
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -f net8.0 --filter "FullyQualifiedName~Impact|FullyQualifiedName~Issue5183" --nologo (134 passed)
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -f net9.0 --filter "FullyQualifiedName~Impact|FullyQualifiedName~Issue5183" --nologo (134 passed)
• make lint
• dotnet run --project tools/CodeIndex.Changelog -- check
• git diff --check
• Fresh Debug build plus root and workspace-manifest index health checks

The mandatory Codex adversarial review ran for the workflow maximum of two rounds. All three first-round findings and both final-round findings were addressed with regression coverage. A reviewer-initiated full Debug .NET 8 run encountered unrelated macOS keychain and temporary-directory permission failures; the focused Release suites above pass on both target frameworks.

Documentation and changelog

• Updated the English and Japanese sections of USER_GUIDE.md, DEVELOPER_GUIDE.md, and TESTING_GUIDE.md.
• Added changelog.d/unreleased/5183.fixed.md.

Follow-ups

None.

Fixes #5183