Work out a node's declared packages and their difference with the mirror in the SDK, so each engine writes its own key (ADR 0565 clause 6, issue 1953)

The engine composes its node's wants itself, so the derivation the controller keeps
privately moves beside the layout: DeclaredPackagesOf gives the mirror's bytes from the
compositions, DiffDeclaredPackages what the node seat's verb packages compares until the
lease write, and EnginePackagesRead names the state as readers declare it (mesh-host.packages,
as mesh-host.data). Each engine may read the controller mirror's key of its own node alone.

Claude-Session: https://claude.ai/code/session_01NcbgfJ9Gum47zkFKWRaovL

go/buscompose/engine.go

@@ -74,9 +74,11 @@ func engineGrants(p Principal) (pub, sub []string) {
 	if own, err := NodeKeysGrants(EngineDataBucket, p.Node); err == nil {
 		pub = append(pub, own...)
 	}
-	// And its node's declared packages, its own key alone (ADR 0565 clause 6).
+	// And its node's declared packages, its own key alone (ADR 0565 clause 6); and, read, the controller's mirror's key
+	// of its node, which the node seat's verb packages compares it with until the lease write ends the mirror.
 	if safeSubject.MatchString(p.Node) {
 		pub = append(pub, EnginePackagesGrants(p.Node)...)
+		pub = append(pub, readGrants(DeclaredPackagesBucket, p.Node)...)
 	}
 	return append(pub, tp...), append(sub, ts...)
 }