Fix release assembly versioning

[RejectKid]

What changed

• apply the resolved release version during every compilation and pack step so package, assembly, file, and informational versions agree
• keep CI package builds version-consistent and prevent benchmark discovery from rebuilding packaged binaries with default metadata
• validate all three library DLLs and the source-generator DLL inside each NuGet package
• prepare corrective release 1.6.1 and update package documentation

Root cause

The 1.6.0 workflow compiled with the repository's stale VersionPrefix of 1.3.0, then supplied PackageVersion=1.6.0 only to a later dotnet pack --no-build. NuGet therefore contained a correctly versioned package wrapping DLLs whose assembly, file, and informational metadata remained 1.3.0.

Validation

• confirmed the new validator rejects the published 1.6.0 package because BitsKit.Generator.dll is 1.3.0.0
• built and validated 1.6.1: all shipped DLLs report assembly/file version 1.6.1.0 and product version 1.6.1+<commit>
• built and validated 0.0.0-ci.999 to cover prerelease-version handling
• 4,616 tests passed on .NET 8 and .NET 10
• benchmark discovery succeeded with -NoBuild
• release and symbol packages were created successfully

After merge, tag the resulting master commit as v1.6.1 to publish the corrected package.