Add CI guard for release build analyzer regressions

[Widthdom]

Summary

• Add a ubuntu net8.0 CI guard that runs the documented Release solution build with shared compilation disabled.
• Add an internal changelog fragment for the CI regression guard.

Validation

• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet build CodeIndex.sln --configuration Release --no-restore -p:UseSharedCompilation=false
• dotnet format whitespace CodeIndex.sln --verify-no-changes --no-restore --verbosity minimal
• make lint
• dotnet build src/CodeIndex/CodeIndex.csproj
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json

Documentation and Changelog

• Added changelog.d/unreleased/2791.internal.md.
• No docs changes were needed because the documented Release build command remains unchanged; this PR adds CI coverage for it.

Review

• Adversarial review: No blocking/actionable issues found.

Follow-up Candidates

• None.

Fixes #2791