Fix checksum hex case drift

[Widthdom]

Summary

• Enforce ordinal, case-sensitive checksum matching when resolving explicit project-local DB samples.
• Standardize the NameFold fingerprint emitter on lowercase hex.
• Document the lowercase SHA-256 hash convention and add focused regression coverage.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~DbPathResolverTests|FullyQualifiedName~FileIndexerTests|FullyQualifiedName~NameFoldTests" -p:UseSharedCompilation=false
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release --filter "FullyQualifiedName~DbPathResolverTests|FullyQualifiedName~FileIndexerTests|FullyQualifiedName~NameFoldTests" -p:UseSharedCompilation=false --no-restore
• dotnet run --project tools/CodeIndex.Changelog -- check
• codex exec adversarial review: No blocking/actionable issues found.

Full dotnet test CodeIndex.sln -c Release -p:UseSharedCompilation=false was attempted but was stopped after it ran without completion for an extended period in this local environment.

Documentation / Changelog

• Updated DEVELOPER_GUIDE.md.
• Added changelog.d/unreleased/1722.fixed.md.

Follow-up candidates

• None.

Fixes #1722