feat: add thread-safe weak container

[AllDmeat]

Summary

Adds a new DBThreadSafeWeakContainer to DBThreadSafe-ios for thread-safe storage of weak references with an API that matches the library style. This is needed to move the app's local weak container implementation into the shared package.

Key changes

• Added a public DBThreadSafeWeakContainer<T> that supports weak references to class-bound protocol existential types
• The container supports the same lock backends as the main package: Synchronization.Mutex on supported OS versions and pthread_rwlock_t as a fallback
• The weak container exposes only the withLock access API, plus lockType and initializers with lock backend selection
• Added package tests covering weak reference storage, automatic nil after deallocation, support for protocol existential types, and backend selection
• Updated the README with documentation for the new container

Additional changes

• None