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/chain/chain.go

@@ -182,8 +182,20 @@ type Chain struct {
 	Rollouts    Bucket[Rollout]
 	Assignments Bucket[Assignment]
 
-	rollouts Backend
-	holds    Holds
+	rollouts    Backend
+	assignments Backend
+	holds       Holds
+}
+
+// RemoveAssignment ends an assignment's key with a marker, at the revision it was read at, only by a controller that holds
+// the lease. **The one removal the chain has**, for an unassignment in the delivery that moves the assignments: the
+// key's earlier values stay (history), and nothing is purged. The delivery after it keeps an unassigned key with an end
+// mark in its place and removes this (issue 1096). Builds and rollouts have none.
+func (c *Chain) RemoveAssignment(ctx context.Context, key string, revision uint64) error {
+	if err := c.Assignments.guard(ctx, key, nil); err != nil {
+		return err
+	}
+	return c.assignments.Remove(ctx, key, revision)
 }
 
 // RolloutsOf is the rollouts bucket typed as the controller's plan, which carries more than the chain's Rollout names:
@@ -197,6 +209,7 @@ func New(builds, rollouts, assignments Backend, holds Holds) *Chain {
 		Rollouts:    Bucket[Rollout]{rollouts, holds},
 		Assignments: Bucket[Assignment]{assignments, holds},
 		rollouts:    rollouts,
+		assignments: assignments,
 		holds:       holds,
 	}
 }