`serverName` is baked irrevocably into every user and room id, so a hive
that rebuilds onto a new default is a *different homeserver*, not a
renamed one: existing accounts and rooms are stranded, and reverting the
config does not undo it. Its neighbours (`gatewayHost`, the forge domain)
are routing, rediscovered through `.well-known` and fixed by editing them
back. Same diff shape, three orders of magnitude apart in blast radius --
which is an asymmetry a module should carry rather than an operator.
An activation script and not `warnings`, which is where this obviously
belongs and does not work: the condition needs the host filesystem, and
`nixos-rebuild switch --flake` evaluates purely, where
`builtins.pathExists "/var/lib/..."` answers false rather than throwing.
A `warnings` entry gated on it would evaluate, deploy, and print nothing
on every real deployment.
Rendered only when `serverName` is null, so a pinned hive has no script
rather than a script that stays quiet -- a guard that cries wolf at a
correctly-configured deployment makes the next real one read as noise.
Never fails the activation: it warns about a choice that cannot be
undone, and refusing the rebuild of a hive that already chose
deliberately is the opposite of helping.
The probed path is read out of the container's own evaluated config
rather than hardcoded. A guessed path resolves cleanly and silently never
matches, which is the same failure this guard exists to catch one level
up.
b5a631c3a3