feat: add demand-driven field observation

[sonmbol]

Summary

Introduces an argument-free, demand-driven observation mode:

@KMPObservable
extension ArticleViewModel: @retroactive KMPStaticallyObservable {}

A typed StateFlow collector is created only when its projected property is first read, for example $viewModel.articleState. Swift provides the field key path through compile-time dynamic-member lookup, so this requires no runtime reflection, generated source, or imported-member enumeration.

Architecture

• preserves the existing explicit fields: macro and eager observation behavior
• shares one collector per model identity and demanded key path
• lazily allocates field observation slots
• retains field-level Swift Observation invalidation
• cancels field leases on rebinding and teardown
• keeps Kotlin as the authoritative value store
• adds no task or allocation to the established explicit SKIE emission path

Current API tradeoff

The demand-driven path intentionally uses the projected store:

Text($viewModel.articleState.title)

Plain viewModel.articleState still returns the imported Kotlin model directly and cannot intercept its getter without exporter-generated Observation instrumentation. This draft is intended for reviewing that API and the lifetime/performance design before it is considered stable.

Validation

• 48 tests pass (1 platform-specific test skipped on macOS)
• strict concurrency enabled
• warnings treated as errors
• macro expansion coverage
• lazy-start and shared-collector tests
• package-manifest consistency
• public API compatibility check against the baseline
• git diff --check

Closes #14