Keep the assignments and each module's declared config on the bus

mesh-config resolves a module's config values per node from which module runs
where and what each registered module declares, and both lived only in the
controller's store (hq issue 652, design 57 F3). The controller now keeps two
record mirrors, mesh-controller_assignments (<node>.<module>) and
mesh-controller_declared-config (<module>), built from the store by the lease
holder every ten seconds and written only where a value differs, by
compare-and-set, with one assignment-changed or declared-config-changed per
write and a purge said as removed (ADR 0289, design 54 §2-3). A module reads
them through `reads` as mesh-controller.<mirror>; the writers table keeps every
other writer out. Neither holds a secret: an assignment names its node and
module, a declaration is what the public manifest declares.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/link/events.go

@@ -81,6 +81,11 @@ const (
 	// steps from. Said after every save of the serving controller; a delivery's holder also asks for the
 	// walks it follows, so a save made elsewhere is found by comparison, never lost.
 	KeyPlanMoved = "plan-moved"
+	// KeyAssignmentChanged and KeyDeclaredConfigChanged: a key of the controller's assignments or declared-config
+	// mirror was written or removed (novox/hq ADR 0289, design 54 §3, design 57 F3): which key, how and at which
+	// revision of the bucket — never the value, which the bucket holds.
+	KeyAssignmentChanged     = "assignment-changed"
+	KeyDeclaredConfigChanged = "declared-config-changed"
 )
 
 // Applied is what a machine now runs, as the mesh states it.