A swarm runs one homeserver, so its identity belongs to the swarm and
not to whichever hive happens to host it -- otherwise relocating the
container between hives reads as a different homeserver rather than a
move. `gatewayHost` moved for that reason a commit ago; `serverName`
was left behind, which made the identity hive-shaped and the routing
swarm-shaped.
⚠️ This is the one value on the page that a rebuild cannot undo.
`gatewayHost` is a routing detail clients rediscover through
`.well-known`; `server_name` is embedded in every user and room id, so
adopting a new one strands the existing accounts and rooms rather than
renaming them -- their ids still name a homeserver that stops
answering. Moving the DEFAULT is safe; moving a RUNNING deployment is
not, and existing hives must pin the value they already minted ids
under before rebuilding.
The fallback stays total on a null swarm domain for the same reason as
its neighbours: the required-domain assertion in hive-network.nix is
what should fire, not a coercion error from an unrelated option
interpolating null.
The legacy-pin eval probe now covers `serverName` as well. It existed
to answer "what do I set so old deployments don't change", and was
proving that only for the two values that are cheap to change -- the
irreversible one was the one it did not assert.
a5210311bf