runtime: extract a generic supervised-component primitive from the adapter actor

[mfw78]

Extract the generic supervised-component and host-actor machinery out of AdapterActor into nexum-runtime, and collapse the hardcoded kind dispatch into a generic role loop. The supervisor currently hardcodes component kinds via enum ModuleKind { EventModule | VenueAdapter } with a match kind dispatch, and AdapterActor is a special-cased provider actor.

Why

Hardcoding component kinds and special-casing the adapter actor bakes venue shapes into the core and leaves provider components outside the recovery machinery. Extracting the reusable primitive (fuel refuel, trap-to-error projection, async-mutex serialization, and restart/poison-sweep membership) lets any provider component join the sweeps and lets the supervisor dispatch generically instead of by named kind. Part of milestone M2: Generic venue-agnostic host. Blocked by: Grow the Extension seam to carry worker/provider roles. See docs/design/videre-split-plan.md and docs/design/issue-milestone-plan.md.

Scope

• Extract the generic supervised-component and host-actor primitive from AdapterActor into nexum-runtime: fuel refuel, trap-to-error projection, async-mutex serialization, and sweep membership.
• Collapse the match kind dispatch into a generic role loop.
• Fold provider components into the restart and poison sweeps so they expose adapters_alive/providers_alive.
• Remove any venue-named kind arm from the core.

Done when

• The generic supervised-component and host-actor primitive is extracted from AdapterActor into nexum-runtime (fuel, trap projection, serialization, sweeps).
• The supervisor match kind is collapsed to a generic role loop.
• Provider components join the restart and poison sweeps and expose a liveness query.
• No venue-named kind arm remains in the core.

[mfw78]

Landed via #441 (squash-merged into dev/m1). Closing manually since the merge target is not the default branch.