Keep assignments in the assignments bucket and move the rows there at start (issue 1093)

Once chain_moved records the kind, Assign, Unassign and Assigned write and read the keys <node>.<module>, and every join of the store that read assignments is made from the bucket: where a module runs, what is behind, what a machine awaits, a given value due for replacement, taking a module, and whether a module may be forgotten (refused while a standing assignment names it). An unassignment ends the key with a marker and keeps its earlier values, as it removed the row; no ended or pending mark is written yet. The generation rises with each change, and a seat held by an assignment goes with it: the foreign key from seat_holding is dropped by migration 0104 and the controller says both. The provisioner's plan is kept in the values by a pass of the tick that writes that one field by compare-and-set; the assignment-changed event is said from what the bucket holds, whoever wrote it. The start moves the rows with what each machine was last sent, keeping a plan the mirror put in a value.

internal/broker/writers.go

@@ -193,8 +193,9 @@ var WritersTable = []WriterRow{
 	// The record mirrors (novox/hq ADR 0289 §9, design 54 §2): rows of this table, so a grant that lets
 	// anything else write them is refused at composition. Built so far: the assignments and each registered
 	// module's declared config (design 57 F3), and the list of predefined keys kept (design 57 K3).
-	{State: "the assignments mirror", Writer: "controller (lease holder), from its store, only on a change",
-		KeptIn: "key-value " + AssignmentsBucket, Others: "read through `reads` as mesh-controller." + MirrorAssignments,
+	{State: "the assignments mirror", Writer: "controller (lease holder): the verbs that assign, and the sends that record what runs; " +
+		"a mirror of its store only until the assignments are moved to the bus (novox/hq ADR 0408)",
+		KeptIn: "key-value " + AssignmentsBucket + ", the record once moved", Others: "read through `reads` as mesh-controller." + MirrorAssignments,
 		Subjects: kvOf(AssignmentsBucket), Writes: isController},
 	// The chain (novox/hq ADR 0408): builds and rollouts beside the assignments above, which keep their bucket. Not
 	// mirrors of the store but the record, written by the lease holder by compare-and-set and removed by nobody.