fix: insert above node, clear value history cooldown, search context menu

- Insert 4/8 now inserts above the right-clicked node and shifts siblings
  down instead of appending at end. Insert key shortcut (Shift+Ins = 4,
  Ins = 8). Falls back to append when clicking empty space.
- Clear Value History uses a 5-cycle cooldown counter so heat stays gone
  for ~1s instead of returning on the next async refresh.
- Right-click Search defers showFindBar via QTimer::singleShot so focus
  isn't stolen by the closing context menu.

src/editor.h

@@ -80,6 +80,7 @@ signals:
     void inlineEditCancelled();
     void typeSelectorRequested();
     void typePickerRequested(EditTarget target, int nodeIdx, QPoint globalPos);
+    void insertAboveRequested(int nodeIdx, NodeKind kind);
 
 protected:
     bool eventFilter(QObject* obj, QEvent* event) override;