Inside the official image every named profile has an s6 slot that the container's boot
registers DOWN (multiplex-only). `implicit_multiplex_blocker` still called
`_host_supports_migration`, whose s6 branch refused unconditionally ("Restart the container"),
so a Hermes Cloud host with the key UNSET booted standalone after an in-place update and every
other profile's bot went silent, while an explicit `true` bypassed the guard and worked. The
guard was vetoing a second gateway that could not exist.
- Only a named slot that is UP is a blocker; registered-down slots never veto the default.
`hermes gateway migrate --multiplex` (and the `hermes update` hook) now fold an UP slot
in-process: `s6-svc -d` + `down` file, root slot (re)started, no container restart. Boot
and migration share ONE fold rule (`gateway_multiplex_s6.fold_named_slot_intent`).
- A multi-profile host that resolves standalone on a guard prints a boxed warning at gateway
start, in the `hermes update` summary, in `hermes gateway status`, and the dashboard
`/api/status` carries `multiplex_standalone_reason` with a banner. Single-profile installs
are not warned.
- The resolved unset→on default is written as `gateway.multiplex_profiles: true` into the
default profile's config.yaml (comment-preserving writer, once, never on a guard refusal).
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