runtime: extract the venue registry service and delete the privileged router field

[mfw78]

Turn the privileged HostState.pool_router field into an extension-owned HostService carried through the generic services map, and delete the named supervisor field. The router is currently a privileged field HostState.pool_router in host/state.rs:54, built and cloned through supervisor.rs.

Why

Once the seam can carry a service, the router no longer needs to be a privileged field: videre can register it as the renamed, un-privileged VenueRegistry behind HostState.services[ns]. Deleting HostState.pool_router and still booting the echo-venue is the forcing function that proves the host layer is intent-free. This issue covers the host half, holding the router only through the generic services map and removing the named field; the VenueRegistry implementation itself belongs to the extension. 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

• Delete the HostState.pool_router field.
• Carry the router only via HostState.services as a type-erased HostService, renamed VenueRegistry.
• Remove every PoolRouter symbol from nexum-runtime/src.
• Keep the echo-venue booting and routing a submit through the services map.

Done when

• The HostState.pool_router field is deleted.
• The router is carried only via HostState.services as a type-erased HostService renamed VenueRegistry.
• No PoolRouter symbol remains in nexum-runtime/src.
• The echo-venue still boots and routes a submit.

[mfw78]

Landed via #443 (squash-merged into dev/m1). Closing manually since the merge target is not the default branch.