Two concurrent gateway sessions calling memory add/replace/remove
simultaneously could both read the old state, apply their changes
independently, and write — the last writer silently drops the first
writer's entry.
Fix: wrap each mutation in a file lock (fcntl.flock on a .lock file).
Under the lock, re-read entries from disk to get the latest state,
apply the mutation, then write. This ensures concurrent writers
serialize properly.
The lock uses a separate .lock file since the memory file itself is
atomically replaced via os.replace() (can't flock a replaced file).
Readers remain lock-free since atomic rename ensures they always see
a complete file.
d81de2f3d8