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Every merge opened a walk and the next merge of the branch superseded it: two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 One walk at a time; a started walk is never superseded, a waiting one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

@@ -40,6 +40,32 @@ type merges interface {
 	AnnouncedMerges(ctx context.Context, since time.Time) ([]link.AnnouncedMerge, error)
 }
 
+// unheardMerges is the announcements of merges the controller has not heard (novox/hq ADR 0276): a merge kept
+// in a batch is not acted on until its batch is cut, which can be past mergeGrace behind a long walk, and is
+// not missed for that.
+type unheardMerges struct {
+	merges
+	inv *inventory.Inventory
+}
+
+func (u unheardMerges) AnnouncedMerges(ctx context.Context, since time.Time) ([]link.AnnouncedMerge, error) {
+	all, err := u.merges.AnnouncedMerges(ctx, since)
+	if err != nil {
+		return nil, err
+	}
+	var out []link.AnnouncedMerge
+	for _, a := range all {
+		_, kept, err := u.inv.MergeOf(ctx, a.Owner+"/"+a.Repo, a.Commit)
+		if err != nil {
+			return nil, err
+		}
+		if !kept {
+			out = append(out, a)
+		}
+	}
+	return out, nil
+}
+
 // catchingUpOnMerges reads back the forge's announcements on a timer, until the context ends.
 func catchingUpOnMerges(ctx context.Context, open *stores, announced merges) {
 	f := following{open}
@@ -61,7 +87,7 @@ func catchingUpOnMerges(ctx context.Context, open *stores, announced merges) {
 		case <-tick.C:
 		}
 		watchedMerges.begin()
-		err := catchUpOnMerges(ctx, time.Now(), announced, catalogued, f.SourceMoved, func(format string, args ...any) {
+		err := catchUpOnMerges(ctx, time.Now(), unheardMerges{announced, open.inventory}, catalogued, f.SourceMoved, func(format string, args ...any) {
 			fmt.Printf(format+"\n", args...)
 		})
 		// What the pass found is what S5 says (novox/hq to-be 45 §3); a pass that could not read