Add ReentrantLock to LlmModule, matching Module.java pattern

Per reviewer feedback: the lock catches accidental unsafe concurrency
from app code (double-tap, rotation during generation, missed stop()).
It serializes access to non-thread-safe native state and turns silent
native crashes into clear Java exceptions.

Pattern matches Module.java exactly:
- generate()/prefill()/load()/resetContext() use lock()
- close() uses tryLock() — fails fast, no ANR risk
- stop() stays lock-free (native atomic flag for cross-thread interrupt)

Convenience generate() overloads delegate to terminal methods that
hold the lock — the checkNotDestroyed() inside the lock is the
correct check point (no TOCTOU race).

This commit was authored with the help of Claude.

Author: psiddh