feat: notifications say what changed and alert once: a grouped notice names the tracks, grows without re-alerting, and a phone pile lists its titles (M542 #6146-#6151)

- An update to a grouped notice keeps its arrival time and email state, so the
  phone and the email no longer go off again each time it grows.
- Notices carry the names of what changed (newest ten), shown as three and
  "and N more": tracks still missing after the scan's move/replacement fold,
  duplicate groups new to a sweep, songs merged automatically, and the track
  and listener a playback error was for.
- Duplicates found counts only the groups new to that sweep; playback errors
  count reports, not the unresolved total.
- A scan failure says what stopped and links to Admin > Library.
- Android: a pile of more than three alerts lists the newest five titles.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/notifications/render_test.go

@@ -45,6 +45,36 @@ func TestRender_CoalescedCountsReadNaturally(t *testing.T) {
 	require.Equal(t, "Some tracks went missing", Render(KindTracksMissing, []byte(`{}`)).Title)
 }
 
+// M542: a notice says which, not only how many.
+func TestRender_NamesWhatChanged(t *testing.T) {
+	require.Equal(t, "No longer on disk: Weezer – Buddy Holly.",
+		render(KindTracksMissing, Payload{Count: 1, Items: []string{"Weezer – Buddy Holly"}}).Body)
+	require.Equal(t, "No longer on disk: A – 1 and B – 2.",
+		render(KindTracksMissing, Payload{Count: 2, Items: []string{"A – 1", "B – 2"}}).Body)
+	// Three are named; the rest of the count, not only the rest of the
+	// stored names, is "more".
+	require.Equal(t, "Held more than once: A, B, C and 9 more.",
+		render(KindDuplicatesFound, Payload{Count: 12, Items: []string{"A", "B", "C", "D", "E"}}).Body)
+	require.Equal(t, "12 new duplicates to review", render(KindDuplicatesFound, Payload{Count: 12}).Title)
+	require.Equal(t, "Extra copies merged into the one the album keeps: A and B.",
+		render(KindDuplicatesResolved, Payload{Count: 2, Items: []string{"A", "B"}}).Body)
+	require.Equal(t, "Failed to play: Weezer – Buddy Holly (alice).",
+		render(KindPlaybackErrors, Payload{Count: 1, Items: []string{"Weezer – Buddy Holly (alice)"}}).Body)
+}
+
+func TestRender_RowsFromBeforeItemsReadAsTheyDid(t *testing.T) {
+	require.Equal(t, "Their files are no longer where the library expects them.",
+		render(KindTracksMissing, Payload{Count: 4}).Body)
+	require.Equal(t, "Merged 8 duplicate copies.",
+		render(KindDuplicatesResolved, Payload{Count: 716, Detail: "Merged 8 duplicate copies."}).Body)
+}
+
+func TestRender_AScanFailureSaysWhatStopped(t *testing.T) {
+	r := render(KindScanFailed, Payload{Count: 1, Detail: "scan root \"/music\" unavailable"})
+	require.Equal(t, `The last scan stopped: scan root "/music" unavailable`, r.Body)
+	require.Equal(t, "/admin/library", r.Link)
+}
+
 func TestRender_AnUnreadablePayloadStillRenders(t *testing.T) {
 	r := Render(KindRequestApproved, []byte(`not json`))
 	require.Equal(t, "Request approved", r.Title)