Clarify definition and impact diagnostics

[Widthdom]

Summary

• Add C# definition disambiguators for partial types, extension methods, and overloads.
• Add impact failure chains, suggestion categories, and strict CLI behavior for unresolved impact analyses.
• Document the new diagnostics in README and DEVELOPER_GUIDE.

Validation

• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -p:UseSharedCompilation=false --no-build --filter "FullyQualifiedName~DbReaderTests"
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -p:UseSharedCompilation=false --no-build --filter "FullyQualifiedName~QueryCommandRunnerTests.RunImpact"
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -p:UseSharedCompilation=false --no-build --filter "FullyQualifiedName~McpServerTests"
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json

Documentation and changelog

• Updated README.md and DEVELOPER_GUIDE.md.
• Added changelog.d/unreleased/1638.changed.md.
• Added changelog.d/unreleased/2002.changed.md.

Review

• Ran codex adversarial review. It flagged stale-base unrelated deletions after origin/main advanced; merging latest origin/main resolved those from the final diff.

Fixes #1638
Fixes #2002