Give a consumer's max-deliveries key one watcher, counting what was given up (issue 440)

The dead-letter row and a max-deliveries advisory without a token both said
bus.<stream>.<consumer>.max-deliveries: each look added an observation and a
line of evidence through the row, and the two overwrote each other's words.
The row owns the key since issue 330, so the advisory watcher leaves it, and
the row now says when the newest held message was given up, so a letter held
for a day no longer reads as observed every 30 seconds. Times without Happened
is refused, since the raise ignored it and an update counted it.

internal/link/deadletters_test.go

@@ -334,3 +334,33 @@ func TestNoticesThatCannotBeTakenAreSaidAndServingGoesOn(t *testing.T) {
 	}
 	eventually(t, "a report heard while the notices cannot be taken", func() bool { return held.count() == 1 })
 }
+
+// When each consumer last gave up on a message DEAD_LETTERS still holds, for the condition that counts
+// what was given up on rather than how often it was looked at (novox/hq issue 440): the newest kept, by
+// when the server said it gave up.
+func TestTheNewestHeldDeadLetterSaysWhenItWasGivenUp(t *testing.T) {
+	js := aBus(t)
+	first := time.Date(2026, 10, 10, 21, 4, 0, 0, time.UTC)
+	for seq, at := range map[int]time.Time{1: first, 2: first.Add(time.Minute)} {
+		if _, err := KeepDeadLetter(js.Context(), []byte(fmt.Sprintf(`{"stream":"EVENTS","consumer":"media_sonarr",`+
+			`"stream_seq":%d,"deliveries":5,"timestamp":%q}`, seq, at.Format(time.RFC3339Nano)))); err != nil {
+			t.Fatal(err)
+		}
+	}
+	if _, err := KeepDeadLetter(js.Context(), []byte(fmt.Sprintf(`{"stream":"EVENTS","consumer":"media_radarr",`+
+		`"stream_seq":3,"deliveries":5,"timestamp":%q}`, first.Format(time.RFC3339Nano)))); err != nil {
+		t.Fatal(err)
+	}
+	held, err := HeldDeadLetters(js.Context())
+	if err != nil {
+		t.Fatal(err)
+	}
+	newest, err := NewestDeadLetters(js.Context(), held)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(newest) != 2 || !newest["EVENTS.media_sonarr"].Equal(first.Add(time.Minute)) ||
+		!newest["EVENTS.media_radarr"].Equal(first) {
+		t.Fatalf("%v", newest)
+	}
+}