Add Singleton lifecycle diagnostics and async initialization

[Skymly]

Summary

• add async Singleton initialization through InitializeAsync and generated GetInstanceAsync()
• add Singleton lifecycle diagnostics DP067–DP071
• add analyzer/generator/integration coverage and synchronize release tracking
• update build, packaging, CI, and project metadata for the new diagnostics

Validation

• targeted source-generator, analyzer, and Singleton integration tests passed
• ./build.ps1 --target CiPack --configuration Release
• CiPack result: 663 tests passed; package verification and net8.0/net48 consumer smoke builds passed

Closes #224

[Skymly]

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

Local validation also passed: CiPack completed with 663 tests passing, zero failures, package verification succeeded, and both net8.0/net48 consumer smoke builds passed.

The Singleton lifecycle diagnostics and async initialization changes are ready for review.