Fold a pending assignment into the value of its assignment key (issue 1094)

A wait for a build is the value of <node>.<module> with state pending and a pending object (id p-<unix nanoseconds>, a string), so every reader that skips a value with a state skips it, and the controller's own lists skip it through Stands. The transitions are compare-and-set on the key's revision (claim, release, settle, repoint, mark); assigning over a wait carries it into the value that stands, applied when it was waiting; a plan pass never overwrites it; one open wait per machine and module, and a key that holds the module refuses a new one. The table is no longer read or written, ForgetEndedPending is gone (no key is removed), and a wait of a machine the store dropped is none. Callers change type only; their conditions are the next issue.

internal/inventory/chain.go

@@ -147,3 +147,21 @@ func (i *Inventory) BeginSwitchForTest(kind string) {
 		i.moved.switching.Store(true)
 	}
 }
+
+// theChain is the chain this inventory writes and reads, opened if a command has not yet: for what has no table of its own
+// to be read from while it is moved (a pending assignment is a value of the assignments bucket from the start).
+func (i *Inventory) theChain(ctx context.Context) (*chain.Chain, error) {
+	i.moved.mu.Lock()
+	defer i.moved.mu.Unlock()
+	if i.chain == nil && i.moved.open != nil {
+		c, err := i.moved.open(ctx)
+		if err != nil {
+			return nil, fmt.Errorf("%w: %v", ErrNoChain, err)
+		}
+		i.chain = c
+	}
+	if i.chain == nil {
+		return nil, ErrNoChain
+	}
+	return i.chain, nil
+}