feat: raster image render pipeline (IMAGE command + GPU textured quad)

- Add IMAGE / IMAGEATTACH / IM command with async file-picker (rfd)
- src/scene/image_model.rs: ImageModel with RGBA pixel data + quad corners
- src/scene/pipeline/image_gpu.rs: ImageGpu with wgpu texture, sampler, opacity uniform
- src/scene/pipeline/mod.rs: image render pipeline (pass 2, between hatches and meshes)
- src/scene/render.rs: include images in Primitive, call upload_images in prepare
- src/scene/mod.rs: images HashMap, populate_images_from_document, cache on add_entity
- src/modules/home/draw/raster_image.rs: ImageCommand (two-click: origin + width, aspect-correct preview)
- src/io/mod.rs: pick_image_file() async helper (file dialog + dimension decode)
- app: ImagePick / ImagePickResult messages, populate on file open and undo/redo

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/history.rs

@@ -35,8 +35,10 @@ impl H7CAD {
             .filter(|h| self.tabs[i].scene.document.get_entity(*h).is_some())
             .collect::<HashSet<_>>();
         self.tabs[i].scene.populate_hatches_from_document();
+        self.tabs[i].scene.populate_images_from_document();
         self.tabs[i].scene.clear_preview_wire();
         self.tabs[i].scene.meshes.clear();
+        self.tabs[i].scene.images.clear();
         self.tabs[i].active_cmd = None;
         self.tabs[i].snap_result = None;
         self.tabs[i].active_grip = None;