Cap solution project discovery and expansion

[Widthdom]

Summary

• Cap automatic .sln candidate discovery, fallback project traversal, and index --project target materialization.
• Make SolutionProjectResolver fallback traversal long-path-safe and resilient to per-directory IO/permission failures with bounded diagnostics.
• Add focused regression tests plus Developer Guide and changelog fragment updates.

Validation

• dotnet build -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -p:UseSharedCompilation=false
• dotnet format CodeIndex.sln --verify-no-changes
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Codex adversarial review: No blocking/actionable issues found.

Documentation / Changelog

• Updated DEVELOPER_GUIDE.md for solution discovery, fallback traversal, and project expansion limits.
• Added changelog fragments:
  • changelog.d/unreleased/3065.fixed.md
  • changelog.d/unreleased/3066.fixed.md
  • changelog.d/unreleased/3213.fixed.md
  • changelog.d/unreleased/3214.fixed.md

Follow-up Candidates

None.

Fixes #3213
Fixes #3214
Fixes #3066
Fixes #3065