Add ReentrantLock to LlmModule, matching Module.java pattern

Serializes access to non-thread-safe native state. Prevents accidental
concurrent calls (e.g. double-tap, rotation during generation) from
causing native crashes — second caller blocks until the first finishes.

- 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)

This commit was authored with the help of Claude.

Author: psiddh