End an unassigned key with ended and remove no key of the chain (issue 1093)

ADR 0408 section 4 says no key of the chain is removed, a delete marker included. The Backend loses Remove and the Chain RemoveAssignment. An unassignment, and the end of a value only pulled in by another, write the value again with ended set, by compare-and-set; assigning it again writes a fresh standing value over it. A value with ended or a state is no assignment (Assignment.Stands) to the assignment lists, the joins, forgetting a module, seat holdings, the given values, the move and the assignment-changed event.

internal/chain/records.go

@@ -186,7 +186,16 @@ type Assignment struct {
 	Commit   string    `json:"commit,omitempty"`
 	Rollout  string    `json:"rollout,omitempty"`
 	Since    time.Time `json:"since,omitzero"`
+
+	// Ended is when the assignment ended (an unassignment): absent while it stands. **No key of the chain is removed**
+	// (ADR 0408 §4), so an unassigned module keeps its key, saying so; every reader treats a value with Ended, or with a
+	// State, as no assignment. State is, in a later delivery, the pending assignment waiting for its build (issue 1094).
+	Ended time.Time `json:"ended,omitzero"`
+	State string    `json:"state,omitempty"`
 }
 
+// Stands says the value is an assignment that holds: it has not ended and is not waiting in a state of its own.
+func (a Assignment) Stands() bool { return a.Ended.IsZero() && a.State == "" }
+
 // Key is an assignment's key.
 func (a Assignment) Key() string { return a.Node + "." + a.Module }