Add Singleton lifecycle diagnostics and async initialization

[Skymly]

Background

The project currently has Singleton generation and DI lifetime diagnostics, but lacks a cohesive lifecycle model for async initialization and several common double-owner/mutable-state cases.

Scope

• Add GenerateSingletonAttribute.InitializeAsync with Task/ValueTask signature validation (DP067).
• Generate GetInstanceAsync() backed by a lazy async singleton.
• Add analyzer diagnostics for generated-singleton + DI double registration (DP068), non-thread-safe mutable instance state (DP069), mutable static singleton candidates (DP070), and mutable static singleton + DI registration (DP071).
• Register the new diagnostics and add generator, analyzer, integration, packaging, and CI coverage.

Acceptance criteria

• CiPack passes with zero test failures and zero compile warnings/errors.
• NuGet package and net8.0/net48 consumer smoke verification pass.
• Diagnostic IDs and unshipped release tracking remain synchronized.