Let a merge that shares nothing skip the merge window (hq issue 525)

A merge waited the whole window even when nothing assembling shared anything
with it. It now joins only a batch it is related to (ADR 0304 §1, read by ADR
0306 §3: repository, a module the cut would move, a graph edge, a group head),
folding the newer into the oldest when it relates to two. A merge related to
none starts a batch whose window is closed at once (PlanBatch.CutAtOnce),
unless its delivery group may still have a member to merge (ADR 0306 §2).

The controller cannot see a group's unmerged members (that is mesh-delivery's
record), so a merge from any head still waits, in a batch of its own: the safe
error is to wait. Walks stay one at a time; a batch cut at once queues behind
an open walk. A stop's merges walked again join a related batch, or their own.

internal/inventory/plans.go

@@ -117,6 +117,11 @@ type PlanBatch struct {
 	// the batch's walk does not build them.
 	Closed   bool     `json:"closed,omitempty"`
 	Withheld []string `json:"withheld,omitempty"`
+	// CutAtOnce says the batch's window closed when it was made: its merge shares nothing with a batch
+	// assembling and has no delivery group member still to merge, so it waits for no window (novox/hq ADR 0304
+	// decision 1, ADR 0306 decisions 2 and 3). A merge that joins it later opens its window again. It is not
+	// Closed, which is a stop's.
+	CutAtOnce bool `json:"cut_at_once,omitempty"`
 }
 
 // OwnPath says the record is a batch of merges on the controller's own path.