Prepare 0.2.3-preview1 release

[Skymly]

Summary

• prepare 0.2.3-preview1 package metadata and release changelog
• mark Singleton lifecycle diagnostics DP068–DP071 as shipped and update the roadmap
• align maintainer/design documentation with the preview version

Validation

• ./build.ps1 --target CiPack --configuration Release
• package artifacts created for 0.2.3-preview1
• 663 tests passed; package verification and net8.0/net48 consumer smoke builds passed

Closes #226

[Skymly]

CI is fully green: build-test, build-test-net48, samples, and pack all passed.

Local validation passed as well: CiPack produced the three 0.2.3-preview1 packages, 663 tests passed, package verification succeeded, and net8.0/net48 consumer smoke builds passed.

The release metadata and analyzer tracking are ready for merge.