An older merge does not move a source

The forge announces what it finds merged, and an old merge surfacing late moved the recorded head
backwards and rebuilt everything built from that repository, once per old merge. A merge made
before the source was last seen is history; one that says nothing about when is taken as news.
The catalogue now carries when each source was last seen. The bus-records test follows #116:
a module's tools are every one under its own name.

cmd/mesh-controller/upgrades.go

@@ -240,6 +240,13 @@ func (f following) SourceMoved(ctx context.Context, m link.SourceMoved) error {
 		if e.Source.BuiltFrom == m.Commit {
 			continue
 		}
+		// **A merge older than the last look at the source is history, not a move.** The forge
+		// announces what it finds merged, and an old merge surfacing late would otherwise move the
+		// recorded head backwards and rebuild everything built from that repository, once per old
+		// merge (2026-09-28).
+		if isHistory(m.MergedAt, e.Source.Seen) {
+			continue
+		}
 		if err := inv.SourceMoved(ctx, e.Manifest.Module, m.Commit); err != nil {
 			return notNow(err)
 		}
@@ -362,3 +369,16 @@ func standsOnModule(e inventory.Entry, module string, against map[string][]strin
 	}
 	return false
 }
+
+// isHistory is whether a merge made at mergedAt predates the last time the source was seen. A merge
+// with no time on it is taken as news: refusing it would silence a forge that says less.
+func isHistory(mergedAt string, seen time.Time) bool {
+	if mergedAt == "" || seen.IsZero() {
+		return false
+	}
+	at, err := time.Parse(time.RFC3339, mergedAt)
+	if err != nil {
+		return false
+	}
+	return at.Before(seen)
+}