A module may watch a role's events, and the catch-up turns out to be unnecessary

Moving the build outcome onto its role broke the one module that consumes it, and my own
agreement check passed anyway. The catalogue's subscription derived
`mesh.mod.mesh-build-machine.event.built` — a module namespace for a role's event, which no
such module owns — so it started, connected, and its graph stayed empty. The check compared
names, and the names agreed: the build machine does emit `built`. Only the subjects
disagreed, and a subscription that matches nothing is silence.

A consumed name is a module's event unless it names a role, and this package cannot tell by
looking — so whoever resolved the declaration says which, the way it already does for a seat
held or used. A module that watches a role gets the role's event subject and a consumer
filtered on it; watching grants subscribe and nothing else, because hearing what a role
announced is not taking part in it.

The check now compares the two halves that actually have to match — the subject a consumer
subscribes against the subject an emitter publishes — with a case pinning that it catches
this exact confusion. Comparing names was checking the easy half.

**And that answered the open question about catch-up: there is nothing to build.** The
mechanism exists because a queue on the old bus receives only what is published after it is
bound, so everything built before the catalogue existed was announced to nobody. A stream is
a log and a consumer is a position in it: a consumer created afterwards starts at the
beginning, so the builds are simply there. Asked of a real server, since the whole decision
rested on it — three builds published with nothing listening, then a consumer created, and
all three waiting for it.

internal/broker/users.go

@@ -29,6 +29,8 @@ type Declared struct {
 	Holds []Seat
 	// Uses are the seats this module sends to.
 	Uses []Seat
+	// Watches are the seats whose events it consumes.
+	Watches []Seat
 }
 
 // Records is what composing a user list needs to know about the mesh, and nothing more.
@@ -59,7 +61,7 @@ func Users(r Records) ([]Principal, error) {
 			out = append(out, Principal{
 				Kind: KindModule, Node: node, Module: d.Module,
 				Emits: d.Emits, Consumes: d.Consumes, Serves: d.Serves,
-				Holds: d.Holds, Uses: d.Uses,
+				Holds: d.Holds, Uses: d.Uses, Watches: d.Watches,
 			})
 		}
 	}