Multiplex-only is the direction. This key exists so fleets that lost per-profile gateways in
the switch keep working while the remaining gaps (per-profile stop/restart, WhatsApp
bridge/relay on secondaries, dashboard scoping) are closed, and it goes away once they are.
Every surface that names the key now says so through one shared notice
(STANDALONE_DEPRECATION_NOTICE): the install/start refusal hint, per-profile and default
`gateway status`, the migrate plan, and the host boot log (WARNING, not INFO). The user
guide carries a deprecation admonition where the key is introduced and no longer describes
it as opting out "for good".
Also: the Windows cold-start test fixture stubbed profiles_to_serve(multiplex) without the
new include_standalone kwarg (the PR's one red on CI).
9fe737aef2