Normalize NuGet package metadata part names

[Widthdom]

Summary

• Add a repo-local package normalizer that rewrites NuGet core-properties .psmdcp entries to a stable archive path.
• Run the normalizer in the release workflow before package validation, hashing, and publishing.
• Document the package reproducibility boundary and add a bilingual changelog fragment.

Validation

• dotnet build tools/CodeIndex.PackageNormalize/CodeIndex.PackageNormalize.csproj
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~ReleaseWorkflowTests" -p:EnableAotAnalyzer=false
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json

Note: the same focused test without -p:EnableAotAnalyzer=false currently fails on existing Native AOT IL3050 analyzer errors tracked by #2786.

Documentation / Changelog

• Updated DEVELOPER_GUIDE.md.
• Added changelog.d/unreleased/2756.fixed.md.

Follow-up candidates

• Existing dotnet test fails with Native AOT IL3050 analyzer errors #2786 tracks the unrelated IL3050 analyzer failures that block ordinary test execution.

Fixes #2756