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/order_test.go

@@ -3,6 +3,7 @@ package main
 import (
 	"strings"
 	"testing"
+	"time"
 
 	"github.com/novox/mesh-controller/internal/catalogue"
 	"github.com/novox/mesh-controller/internal/inventory"
@@ -90,3 +91,18 @@ func TestAMergeMatchesTheSourcesBuiltFromIt(t *testing.T) {
 		}
 	}
 }
+
+// A merge made before the source was last seen is history: it does not move the source, and a
+// merge that says nothing about when it was made is taken as news.
+func TestAMergeOlderThanTheLastLookIsHistory(t *testing.T) {
+	seen := time.Date(2026, 9, 28, 3, 0, 0, 0, time.UTC)
+	if !isHistory("2026-09-28T02:00:00Z", seen) {
+		t.Fatal("an older merge was taken as news")
+	}
+	if isHistory("2026-09-28T04:00:00Z", seen) {
+		t.Fatal("a newer merge was taken as history")
+	}
+	if isHistory("", seen) || isHistory("2026-09-28T02:00:00Z", time.Time{}) {
+		t.Fatal("a merge or a source with no time on it was refused")
+	}
+}