Assemble merges in a rolling window and walk each batch once (hq ADR 0276, issue 362)

Every merge opened a walk and the next merge of the branch superseded it: two
catalogue merges 18 s apart left a walk no delivery held, and the operator
started it by hand 58 minutes later. A merge now joins the open batch, kept in
the store (migration 0089), which is cut into one walk when no merge came for
merge-window (90 s) or at merge-window-at-most (10 min): one commit per
repository, the latest of its branch, with every file the batch's merges
changed. One walk at a time; a started walk is never superseded, a waiting one
is folded into the next. The walk names every merge it answers on the wire
(delivery.merges, taken_over_by, batch). A failed walk walks its earlier merges
alone, newest first, until one is delivered. A delivery group's order becomes
tier edges inside the walk. plans shows the batch assembling; S18 and S19
bound its waits; S16 names the merges a waiting walk answers.

cmd/mesh-controller/signals_test.go

@@ -142,6 +142,28 @@ var suppressions = map[string]suppression{
 				since: f.now.Add(-31 * time.Minute)}}
 		},
 	},
+	// A batch still assembling past its maximum with no walk open (novox/hq ADR 0276): a minute's grace.
+	"S18": {
+		inside: func(f *signalFacts) {
+			f.batches = []batchFacts{{id: "plan-3", state: inventory.PlanAssembling, grouped: "novox/app@c0ffee11",
+				atMost: f.now.Add(-59 * time.Second), closed: f.now.Add(-59 * time.Second)}}
+		},
+		past: func(f *signalFacts) {
+			f.batches = []batchFacts{{id: "plan-3", state: inventory.PlanAssembling, grouped: "novox/app@c0ffee11",
+				atMost: f.now.Add(-61 * time.Second), closed: f.now.Add(-61 * time.Second)}}
+		},
+	},
+	// A batch queued behind an open walk past that walk's bound, named.
+	"S19": {
+		inside: func(f *signalFacts) {
+			f.batches = []batchFacts{{id: "plan-3", state: inventory.PlanQueued, grouped: "novox/app@c0ffee11",
+				closed: f.now.Add(-59 * time.Minute), behind: "plan-1", walkBound: time.Hour}}
+		},
+		past: func(f *signalFacts) {
+			f.batches = []batchFacts{{id: "plan-3", state: inventory.PlanQueued, grouped: "novox/app@c0ffee11",
+				closed: f.now.Add(-61 * time.Minute), behind: "plan-1", walkBound: time.Hour}}
+		},
+	},
 	// A send refused because it would replace the bus outside its planned step: said at its first refusal,
 	// whatever the bound (novox/hq issue 336). Inside: a new bus build waits, and no send was refused for it.
 	"S17": {