`swarm agent create` failed with "no user store at
swarm-authelia-bridge-users.json but users.yml already holds users" on
any hive that had users. The guard was correct; what was wrong is that
two files both claimed to be canonical for one physical file.
The bridge kept a private users.json and rendered users.yml from it,
while swarmctl kept its own pair against the same users.yml. A writer
whose own JSON was absent could not tell "nothing here yet" from
"someone else's users", so it refused to write at all.
users.yml becomes the store: read before write, through serde_norway
rather than a hand-rolled emitter. Unknown top-level and per-user keys
round-trip through `extra`, or whichever process writes second would
silently delete what the first added. Validation stays on the write path
-- a bare to_string(&store) serialises perfectly and drops the "no
control character ever reaches this file" guarantee silently.
The seed constant goes with the guard: nothing writes a seed now, an
absent file is an empty store, and left as a pub constant it read as if
the seed dance were still load-bearing.
