Phase 3.5a: place imported images on the canvas (image-filled rect)

Replaces the DCEL "not yet supported" stubs so importing/dropping an image actually
puts it in the vector scene.

- AddShapeAction gains an `image_fill` + `AddShapeAction::image_rect(...)` constructor:
  a borderless rectangle (invisible edges) whose enclosed region is paint-bucketed and
  tagged with an image asset id. The renderer already prioritises `image_fill`.
- Direct import (auto_place_asset): an imported image is placed centered on the canvas
  at native size on a vector layer.
- Drag from the asset library onto the stage: image-filled rect at the drop point
  (centered), native size, using the asset's dimensions.

Next: SetImageFillAction + an Info-Panel image-fill picker for existing shapes; then
3.5b container persistence.

lightningbeam-ui/lightningbeam-core/src/actions/add_shape.rs

@@ -23,6 +23,9 @@ pub struct AddShapeAction {
     stroke_color: Option<ShapeColor>,
     fill_color: Option<ShapeColor>,
     is_closed: bool,
+    /// When set, the enclosed region is filled with this image asset (instead of a
+    /// solid colour). The renderer prioritises `image_fill` over colour/gradient.
+    image_fill: Option<Uuid>,
     description_text: String,
     /// Snapshot of the graph before insertion (for undo).
     graph_before: Option<VectorGraph>,
@@ -46,15 +49,53 @@ impl AddShapeAction {
             stroke_color,
             fill_color,
             is_closed,
+            image_fill: None,
             description_text: "Add shape".to_string(),
             graph_before: None,
         }
     }
 
+    /// A borderless, axis-aligned rectangle filled with an image asset — the result
+    /// of importing/dropping an image onto a vector layer.
+    pub fn image_rect(
+        layer_id: Uuid,
+        time: f64,
+        x: f64,
+        y: f64,
+        w: f64,
+        h: f64,
+        asset_id: Uuid,
+    ) -> Self {
+        let mut path = BezPath::new();
+        path.move_to((x, y));
+        path.line_to((x + w, y));
+        path.line_to((x + w, y + h));
+        path.line_to((x, y + h));
+        path.close_path();
+        Self {
+            layer_id,
+            time,
+            path,
+            stroke_style: None, // invisible edges — just the image
+            stroke_color: None,
+            fill_color: None,
+            is_closed: true,
+            image_fill: Some(asset_id),
+            description_text: "Add image".to_string(),
+            graph_before: None,
+        }
+    }
+
     pub fn with_description(mut self, desc: impl Into<String>) -> Self {
         self.description_text = desc.into();
         self
     }
+
+    /// Fill the created region with an image asset (image takes render priority).
+    pub fn with_image_fill(mut self, asset_id: Uuid) -> Self {
+        self.image_fill = Some(asset_id);
+        self
+    }
 }
 
 impl Action for AddShapeAction {
@@ -87,16 +128,25 @@ impl Action for AddShapeAction {
                 DEFAULT_SNAP_EPSILON,
             );
 
-            // Apply fill if this is a closed shape with fill
-            if self.is_closed {
-                if let Some(ref fill) = self.fill_color {
-                    // Compute centroid of the path's bounding box and paint-bucket fill
-                    let bbox = self.path.bounding_box();
-                    let centroid = kurbo::Point::new(
-                        (bbox.x0 + bbox.x1) / 2.0,
-                        (bbox.y0 + bbox.y1) / 2.0,
-                    );
-                    graph.paint_bucket(centroid, fill.clone(), FillRule::NonZero, 0.0);
+            // Apply fill if this is a closed shape with a colour and/or image fill.
+            if self.is_closed && (self.fill_color.is_some() || self.image_fill.is_some()) {
+                // Compute centroid of the path's bounding box and paint-bucket fill.
+                let bbox = self.path.bounding_box();
+                let centroid = kurbo::Point::new(
+                    (bbox.x0 + bbox.x1) / 2.0,
+                    (bbox.y0 + bbox.y1) / 2.0,
+                );
+                // paint_bucket needs a colour; an image-only fill uses a placeholder
+                // that the image overrides (cleared below).
+                let color = self.fill_color.clone().unwrap_or_else(|| ShapeColor::rgba(255, 255, 255, 255));
+                if let Some(fid) = graph.paint_bucket(centroid, color, FillRule::NonZero, 0.0) {
+                    if let Some(asset_id) = self.image_fill {
+                        let fill = graph.fill_mut(fid);
+                        fill.image_fill = Some(asset_id);
+                        if self.fill_color.is_none() {
+                            fill.color = None; // image-only: don't double-paint a colour
+                        }
+                    }
                 }
             }
         }

lightningbeam-ui/lightningbeam-editor/src/main.rs

@@ -4921,15 +4921,22 @@ impl EditorApp {
 
         // Add clip instance or shape to the target layer
         if let Some(layer_id) = target_layer_id {
-            // For images, create a shape with image fill instead of a clip instance
+            // For images, create an image-filled rectangle on the (vector) layer,
+            // centered on the canvas at native size.
             if asset_info.clip_type == panes::DragClipType::Image {
-                // Get image dimensions
-                let (width, height) = asset_info.dimensions.unwrap_or((100.0, 100.0));
-
-                // TODO: Image fills on DCEL faces are a separate feature.
-                // For now, just log a message.
-                let _ = (layer_id, width, height);
-                eprintln!("Image drop to canvas not yet supported with DCEL backend");
+                let (img_w, img_h) = asset_info.dimensions.unwrap_or((100.0, 100.0));
+                let (doc_w, doc_h) = {
+                    let d = self.action_executor.document();
+                    (d.width as f64, d.height as f64)
+                };
+                let x = (doc_w - img_w) / 2.0;
+                let y = (doc_h - img_h) / 2.0;
+                let action = lightningbeam_core::actions::AddShapeAction::image_rect(
+                    layer_id, self.playback_time, x, y, img_w, img_h, asset_info.clip_id,
+                );
+                if let Err(e) = self.action_executor.execute(Box::new(action)) {
+                    eprintln!("Failed to place image: {}", e);
+                }
             } else {
                 // For clips, create a clip instance
                 let mut clip_instance = ClipInstance::new(asset_info.clip_id)

lightningbeam-ui/lightningbeam-editor/src/panes/stage.rs

@@ -11780,9 +11780,18 @@ impl PaneRenderer for StagePane {
                         if let Some(layer_id) = target_layer_id {
                             // For images, create a shape with image fill instead of a clip instance
                             if dragging.clip_type == DragClipType::Image {
-                                // TODO: Image fills on DCEL faces are a separate feature.
-                                let _ = (layer_id, world_pos);
-                                eprintln!("Image drag to stage not yet supported with DCEL backend");
+                                // Image-filled rectangle at the drop point (centered), native size.
+                                let dims = shared.action_executor.document()
+                                    .get_image_asset(&dragging.clip_id)
+                                    .map(|a| (a.width as f64, a.height as f64));
+                                if let Some((img_w, img_h)) = dims {
+                                    let x = world_pos.x as f64 - img_w / 2.0;
+                                    let y = world_pos.y as f64 - img_h / 2.0;
+                                    let action = lightningbeam_core::actions::AddShapeAction::image_rect(
+                                        layer_id, drop_time, x, y, img_w, img_h, dragging.clip_id,
+                                    );
+                                    let _ = shared.action_executor.execute(Box::new(action));
+                                }
                             } else if dragging.clip_type == DragClipType::Effect {
                                 // Handle effect drops specially
                                 // Get effect definition from registry or document