Document ApplicationManager lock ordering and thread-safety contract

[sfloess]

Part of #423: Race condition potential in ApplicationManager concurrent operations

Problem

Lock ordering and thread-safety invariants are not documented, making it easy for future contributors to introduce deadlocks or race conditions during maintenance.

Tasks

• Add class-level Javadoc documenting the lock ordering hierarchy
• Document which methods are thread-safe and which require external synchronization
• Document the thread-safety contract for listener callbacks (which thread, ordering guarantees)
• Add @ThreadSafe / @GuardedBy annotations where appropriate (javax.annotation.concurrent)
• Add a THREADING.md or section in the module README explaining the concurrency model

Acceptance Criteria

• A developer new to the codebase can understand the locking strategy from documentation alone
• Lock ordering is explicitly stated and matches the implementation
• Thread-safety annotations are present on all shared mutable state