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Every merge opened a walk and the next merge of the branch superseded it: two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 One walk at a time; a started walk is never superseded, a waiting one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/delivery_test.go

@@ -227,7 +227,9 @@ func TestAMergeWaitsForItsDeliverysWordAndThePersonsWordWorksWithoutIt(t *testin
 		t.Fatalf("with no holder on record the walk waited (%v) or asked %v", p.Waiting(), *asked)
 	}
 
-	// The holder on record: the next merge waits, asking nothing, and an advance asks nothing either.
+	// The holder on record: the next merge waits, asking nothing, and an advance asks nothing either. One walk
+	// is open at a time (novox/hq ADR 0276): the first ends before the next merge's batch is cut.
+	finish(p.ID)
 	if _, err := inv.Assign(ctx, "anchor", "mesh-delivery"); err != nil {
 		t.Fatal(err)
 	}
@@ -259,10 +261,11 @@ func TestAMergeWaitsForItsDeliverysWordAndThePersonsWordWorksWithoutIt(t *testin
 		t.Fatalf("the word was not kept: %+v %v", got.Delivery, err)
 	}
 
-	// The delivery's owner's own merge never waits for it — and takes over what the older walk had not
-	// built (app, folded in: ADR 0218), which goes with it on the controller's own path.
+	// The delivery's owner's own merge never waits for it. A walk that started is never taken over (novox/hq
+	// ADR 0276): the merge's batch is cut once it ended.
+	finish(p.ID)
 	p = merge("c3cccccccc", "modules/mesh-delivery/main.go")
-	if p.Waiting() || len(*asked) != 4 {
+	if p.Waiting() || len(*asked) != 3 {
 		t.Fatalf("mesh-delivery's own walk waited for mesh-delivery: %v %v", p.Waiting(), *asked)
 	}
 
@@ -280,7 +283,7 @@ func TestAMergeWaitsForItsDeliverysWordAndThePersonsWordWorksWithoutIt(t *testin
 	}
 	advanceHeld(ctx, open)
 	got, _ = inv.PlanByID(ctx, p.ID)
-	if got.Waiting() || !strings.HasPrefix(got.Delivery.By, "a person") || len(*asked) < 5 {
+	if got.Waiting() || !strings.HasPrefix(got.Delivery.By, "a person") || len(*asked) < 4 {
 		t.Fatalf("a person's word did not start the walk: %+v, asked %v", got.Delivery, *asked)
 	}