The third shape: a non-mautrix bridge. It confirms the scaffolding is not
mautrix-specific, and it is laid out differently enough to be worth having -
everything sits directly under the base path rather than in config/ and
data/ subdirectories, the owner and homeserver URL are passed on the command
line rather than through a config file, so the unit is where they can be
checked, and identd optionally binds host port 113.

matrix-base's matrix_server_fqn_* family moved into the shared context;
18 of the roles here read one of them.

Three roles now, three shapes - HTTP receiver, mautrix bridge, non-mautrix
bridge - and the scaffolding took no per-shape special-casing beyond each
role's own variables.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>