feat(state): Add SafeBoxStateManager for better lifecycle support

[harrytmthy]

Summary

This PR introduces SafeBoxStateManager, a dedicated component for coordinating SafeBox lifecycle states and concurrent write management.

It enables:

• ✅ Accurate state transitions (STARTING, WRITING, IDLE, CLOSED)
• ✅ Robust write tracking using atomic counters
• ✅ Safe commit() and apply() sequencing
• ✅ closeWhenIdle() support to defer closing until writes complete

Implementation Details

• SafeBoxStateManager maintains:
  • Atomic write counter to support overlapping commit() / apply() calls
  • CompletableDeferred gating for initial load (launchWithStartingState)
  • Synchronized finalizeWriting() with accurate state emission
• SafeBox now delegates all lifecycle and concurrency decisions to the manager
• closeWhenIdle() is guaranteed to wait for both the initial read and all pending writes

This ensures lifecycle consistency and write safety, especially in async or ViewModel-scoped usage.

Closes #17