Fix: Serialize AppStorage persistence and inject its backend per app

[phranck]

Closes #15.

Summary

• JSONFileStorage captures an immutable cache snapshot for every mutation and persists snapshots through one serial writer queue, so an older snapshot can never overwrite a newer mutation; pending writes complete even if the storage instance is released
• synchronize() is now a true barrier that returns only after every previously issued write has completed; the runtime flushes storage this way during cleanup
• Encode, serialize, and write failures are reported through StoragePersistenceError with sanitized reasons (error domain and code only, never paths or stored content); the default handler logs one line to stderr
• The deprecated mutable StorageDefaults global is gone: every production runtime owns its own JSONFileStorage, and @AppStorage access outside a runtime falls back to a deliberately volatile process-local backend that never touches the file system
• API compatibility manifest and review-policy overrides regenerated from fresh Swift 6.0.3 macOS + Linux snapshots
• State management docs describe the new persistence guarantees

Acceptance criteria from #15

• No older snapshot can overwrite a newer mutation (ordered writer + snapshot tests)
• synchronize() is a true barrier for all prior changes (barrier test)
• Concurrent stress, restart, corruption, and write-failure tests pass on macOS and Linux using only temporary directories
• Two app runtimes do not share storage backends unless explicitly configured
• Persistence errors are observable and contain no path/content leakage (leak-assertion tests)
• Swift 6.0 build, tests, lint, and DocC are warning-free on both platforms (./scripts/test-linux.sh green)

Test plan

• 1332 tests green on macOS (Swift 6.0.3) and Linux (Docker 6.0.3-noble)
• New AppStoragePersistenceTests suite: ordering, barrier, restart, corruption, failure reporting, unbound fallback, concurrency stress
• verify-compatibility-manifest.sh passes against the regenerated manifest