Tell a module where a mesh seat's holder is reached (hq ADR 0222)

The container runtime's module must state the mesh's registry to the runtime it owns, so the
controller can stop writing that into the runtime's file (hq issue 190). ${seat:<seat>:reach}
answers host:port without a binding: nothing required, granted or minted, and the address is
one the mesh already composes into every reference it built. Only mesh-artifact-store is
answered; another seat is refused by name. Unanswered in a file written into as JSON, the empty
member is dropped, so the runtime is never told to trust "".

internal/catalogue/declaration.go

@@ -201,6 +201,11 @@ type Rendering struct {
 	// stored (novox/hq 04-ISSUES/102).
 	ArtifactStore string
 
+	// SeatReach is where this machine reaches the holder of each mesh-scoped seat it may be asked
+	// about (host:port), by seat: what ${seat:<seat>:reach} answers with (novox/hq ADR 0222). Absent
+	// when no holder is reachable yet; see seat_into.go for which seats are answered.
+	SeatReach map[string]string
+
 	// Built is every `<module>/<artifact>` the mesh has built. What tells a reference recorded
 	// with an address — before references were kept without one — from an image a module runs
 	// straight from a public registry.