EntityRegistry.save() called Path.write_text() directly, which truncates
the target file and then writes — so a crash mid-write (power loss, OOM,
filesystem-full mid-flush) leaves an empty or half-written
entity_registry.json. The whole people/projects map is lost; the system
falls back to an empty registry on next load.
Switch to the standard atomic-write pattern: serialize to a sibling
.tmp file in the same directory (so os.replace stays on one filesystem),
fsync, chmod 0o600, then os.replace over the target. The replace is
atomic on POSIX and Windows, so any crash leaves the previous registry
intact instead of a truncated file.
Tests cover: no leftover .tmp on success, and previous content preserved
when os.replace itself raises mid-save.
Arnold Wender
4f36145c2e