PR #30136 review noted the asymmetry: `register_profile_gateway`
used tmp_dir + rename to publish a new service slot atomically,
but the boot-time reconciler wrote files into the slot directly.
Same underlying concern (a concurrent s6-svscan rescan could
observe a half-populated directory), different code path.
Rewrite `container_boot._register_service` to mirror the manager:
build everything in `<scandir>/gateway-<profile>.tmp/`, then
`Path.replace` into place. If a previous interrupted run left a
`.tmp` sibling, it's cleaned up before the new build starts. If
the target already exists, it's removed before the rename so
`Path.replace` doesn't error on a non-empty target (Linux `rename`
overwrites empty targets only).
Three new tests: atomic publication leaves no .tmp leftovers,
overwriting an existing slot still leaves no .tmp leftovers, and
a stale .tmp from an interrupted run is cleaned up automatically.
d0b1ab48dc