Compose each module from its build and read what providers elsewhere serve, pinning mesh-sdk cebc651, so a self-composing node hands its moved modules and settles mailu's domain (issue 1126)

The node read a behind module's source manifest digest and composed every module
from the indexer's raw manifest; frombus now takes the build itself (Scope.KeptAt,
a fingerprint). The reader gives frombus the builder's key, a templates archive
fetched as every archive is (apply.Fetch) once per digest and checked against it,
and another node's served keys one by one, as the node's grant allows them.

cmd/mesh-host/nodeagent.go

@@ -883,7 +883,9 @@ func composerEnv(session *link.Session, node string, say func(string), hooks com
 			return session.AskVault(ctx, "own", args)
 		},
 		// And the vault's word that a secret changed, on which it composes again (ADR 0547 §2).
-		Hear:    session.Hear,
+		Hear: session.Hear,
+		// And a moved module's templates archive, in the artifact store, as every archive is fetched (issue 1126).
+		Fetch:   apply.Fetch,
 		Running: hooks.running, Unseal: hooks.unseal, Refused: hooks.refused, Held: hooks.held}
 }