Module session_lock 

Per-session sentinel lock (Goal 151).

Pulled out of session.rs so the file there focuses on crate::session::SessionWriter / SessionReader semantics. The implementation is unchanged — see the blame on session.rs for historical context. All names re-exported from crate::session so external paths like recursive::session::SessionLock keep working.

Structs

SessionLock

RAII guard preventing concurrent writes to the same JSONL session.

SessionLockBusy

Error type carried inside std::io::Error::other when SessionLock::acquire refuses because another live process holds the lock.