Fix the second review of the origin changes

Stop now counts the origin changes inside a running write, in a new
writingOriginChanges field, and adds them to the count it logs. The prune test
checks which rows survive, including a prefix with rows on both sides of 72
hours. The prefix page tests cover 5,000 and 5,001 rows and a row's time and
peer count. New tests cover the dropped-rows warning and the dropped count's
reset. The prune floor is named minOriginChangesPerPrefix. The README, TODO,
package doc and doc comments are corrected to match.

Model: deepseek-v4-pro

internal/server/prefix_originchanges_internal_test.go

@@ -145,4 +145,75 @@ func TestPrefixDetailSaysWhenMoreThan5000Changes(t *testing.T) {
 	if !strings.Contains(page, "more than 5000 changes") {
 		t.Errorf("page does not say there are more than 5000 changes")
 	}
+
+	if got := strings.Count(page, "<td>Announced</td>"); got != 5000 {
+		t.Errorf("page shows %d change rows, want 5000", got)
+	}
+}
+
+// TestPrefixDetailShowsNoNoteAtExactly5000Changes checks that a prefix with
+// exactly 5,000 origin changes shows all 5,000 and no note.
+func TestPrefixDetailShowsNoNoteAtExactly5000Changes(t *testing.T) {
+	t.Parallel()
+
+	prefix := netip.MustParsePrefix(testPrefixV4)
+
+	changes := make([]routetable.OriginChange, 0, 5000)
+	for i := range 5000 {
+		changes = append(changes, routetable.OriginChange{
+			Prefix: prefix, OriginASN: 64500, Announced: true, PeerCount: 1,
+			Time: testRouteTime(i),
+		})
+	}
+
+	s := newPrefixDetailServer(t, changes)
+
+	rec := httptest.NewRecorder()
+	s.router.ServeHTTP(rec, httptest.NewRequestWithContext(
+		t.Context(), http.MethodGet, "/prefix/"+testPrefixV4, nil))
+
+	if rec.Code != http.StatusOK {
+		t.Fatalf("status %d, want %d; body %s", rec.Code, http.StatusOK, rec.Body)
+	}
+
+	page := rec.Body.String()
+
+	if strings.Contains(page, "more than") {
+		t.Errorf("page says there are more changes, but a prefix has 5000")
+	}
+
+	if got := strings.Count(page, "<td>Announced</td>"); got != 5000 {
+		t.Errorf("page shows %d change rows, want 5000", got)
+	}
+}
+
+// TestPrefixDetailShowsOriginChangeTimeAndPeerCount checks that a row shows
+// its time and its peer count.
+func TestPrefixDetailShowsOriginChangeTimeAndPeerCount(t *testing.T) {
+	t.Parallel()
+
+	prefix := netip.MustParsePrefix(testPrefixV4)
+
+	s := newPrefixDetailServer(t, []routetable.OriginChange{
+		{Prefix: prefix, OriginASN: 64500, Announced: true, PeerCount: 7,
+			Time: testRouteTime(5)},
+	})
+
+	rec := httptest.NewRecorder()
+	s.router.ServeHTTP(rec, httptest.NewRequestWithContext(
+		t.Context(), http.MethodGet, "/prefix/"+testPrefixV4, nil))
+
+	if rec.Code != http.StatusOK {
+		t.Fatalf("status %d, want %d; body %s", rec.Code, http.StatusOK, rec.Body)
+	}
+
+	page := rec.Body.String()
+
+	if !strings.Contains(page, "2026-01-02 03:09:05") {
+		t.Errorf("page does not show the origin change's time")
+	}
+
+	if !strings.Contains(page, "<td>7</td>") {
+		t.Errorf("page does not show the origin change's peer count")
+	}
 }