Valid session handle.
Optionaloptions: number | AbortSignal | BackendReadContext | FileLockOptions
Optional timeout, cancellation signal, lock options, or read context (EXEC-001-T09c).
Object with ok: true and { token } on success, or { ok: false, error } if busy/expired.
Atomically acquires turn ownership on an idle session record.
Checks cancellation signals and execution budget before lock acquisition, during the lock wait, and across synchronous boundaries immediately after write. Atomic file writes (
atomicWriteviafs.rename) represent uninterruptible OS operations; if cancellation occurs across a write boundary, rollback to idle is attempted and turn ownership is revoked. If rollback write fails due to OS I/O error, the record remains until stale recovery reclaims it.