Release 3.8.2.5: cheaper idle UI poll and tighter hot paths.

Delta getState by rev, pause when hidden, cancellable background loops,
hostCache bounds, unlock PollUI around engine status, corebin singleflight,
PingAll worker pool.

Co-authored-by: Cursor <cursoragent@cursor.com>

internal/apphost/handler_test.go

@@ -50,6 +50,33 @@ func TestHTTPBridgeGetState(t *testing.T) {
 	}
 }
 
+func TestHTTPBridgeGetStateUnchanged(t *testing.T) {
+	cfg := &config.Config{
+		Active: "demo",
+		Profiles: []config.Profile{
+			{Name: "demo", Protocol: config.ProtocolNaive, Proxy: "https://u:p@example.com"},
+		},
+	}
+	mgr, err := core.NewManager("", cfg, logbuf.New(0))
+	if err != nil {
+		t.Fatal(err)
+	}
+	a := New(mgr, "", logbuf.New(0))
+	a.APIToken = "test-token"
+
+	full, err := a.GetStateSince("")
+	if err != nil || full.Unchanged || full.Rev == "" {
+		t.Fatalf("full: unchanged=%v rev=%q err=%v", full.Unchanged, full.Rev, err)
+	}
+	again, err := a.GetStateSince(full.Rev)
+	if err != nil || !again.Unchanged || again.Rev != full.Rev {
+		t.Fatalf("delta: %+v err=%v", again, err)
+	}
+	if len(again.Profiles) != 0 {
+		t.Fatalf("unchanged payload should omit profiles, got %d", len(again.Profiles))
+	}
+}
+
 func TestHTTPBridgeRejectsBadToken(t *testing.T) {
 	cfg := &config.Config{}
 	mgr, err := core.NewManager("", cfg, logbuf.New(0))