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Every merge opened a walk and the next merge of the branch superseded it: two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 One walk at a time; a started walk is never superseded, a waiting one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

internal/inventory/durations.go

@@ -125,6 +125,9 @@ func planTierLeft(ctx context.Context, tx pgx.Tx, p Plan, now time.Time) (entere
 	if err != nil {
 		return time.Time{}, err
 	}
+	if oldState == PlanAssembling || oldState == PlanQueued {
+		return now, nil // a batch cut into a walk enters its first tier now (novox/hq ADR 0276)
+	}
 	wasOpen := oldState == PlanBuilding || oldState == PlanRolling
 	if !wasOpen || (oldTier == p.Tier && p.Open()) {
 		return since, nil // still in the tier, or already ended