feat: opt-in CCE_PAINT_WALK single-paint-path rendering

Implement Application::display_list() gated on the CCE_PAINT_WALK env var, returning scene::painter::paint_tree(root_window). Lets the Phase 3 single paint path (one traversal -> one GPU-scissor-clipped DisplayList) be A/B-toggled against the legacy render. Off by default.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

 src/main.rs | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/src/main.rs b/src/main.rs
index 66a78cc..ce5521a 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1327,6 +1327,17 @@ impl Application for GraphApp {
         quads.extend(self.root_window.all_rounded_quads(&self.ui_context));
     }
 
+    fn display_list(&mut self) -> Option<cce_ui::scene::paint::DisplayList> {
+        // Opt-in A/B toggle for the Phase 3 single paint path: with CCE_PAINT_WALK set, render the
+        // widget tree via scene::painter (one traversal -> one clipped DisplayList, drawn with GPU
+        // scissor) instead of the legacy all_rounded_quads geometry. Default off => legacy path.
+        if std::env::var("CCE_PAINT_WALK").is_err() {
+            return None;
+        }
+        let root: *mut (dyn cce_ui::widget::Element + 'static) = self.root_window.as_ptr_mut();
+        Some(cce_ui::scene::painter::paint_tree(&self.ui_context, root))
+    }
+
     fn text_items(&self) -> &[TextItem] {
         &self.text_items
     }