feat: custom arrow tooltip with transparent background

Rewrite RcxTooltip to use WA_TranslucentBackground with a single
contiguous QPainterPath (rounded rect + arrow notch). Pre-set the
DarkTitleBar property to prevent DarkApp from calling
DwmSetWindowAttribute which breaks layered window compositing.

Dismiss all popups (including arrow tooltip) on alt-tab via
MainWindow::changeEvent(ActivationChange).

src/editor.h

@@ -159,6 +159,7 @@ private:
     QWidget* m_historyPopup = nullptr;  // ValueHistoryPopup (file-local class in editor.cpp)
     QWidget* m_disasmPopup = nullptr;        // TitleBodyPopup (file-local class in editor.cpp)
     QWidget* m_structPreviewPopup = nullptr; // TitleBodyPopup (file-local class in editor.cpp)
+    QWidget* m_arrowTooltip = nullptr;       // RcxTooltip (arrow callout)
     const Provider* m_disasmProvider = nullptr;   // snapshot or real — for reading tree data
     const Provider* m_disasmRealProv = nullptr;   // real process provider — for reading code at arbitrary addresses
     const NodeTree* m_disasmTree = nullptr;