Prune ghost _index.json rows whose backing session file no longer exists, on both incremental index writes and all_sessions() reads. Fixes duplicate session entries after session-id rotation (e.g. context compression). Also pre-snapshots in_memory_ids under a single LOCK acquisition in all_sessions() rather than one per row.
Closes#846.
Review additions: optimised lock pattern in all_sessions() (one LOCK acquisition instead of N). Tests: 1856 passing.