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/mod.rs

@@ -397,6 +397,11 @@ pub enum Message {
     // Field edit messages:
     DsEdit(DsField, String),
     DsToggle(DsField),
+    // ── Raster Image ──────────────────────────────────────────────────────
+    /// Open file-picker dialog for IMAGE command (async).
+    ImagePick,
+    /// Result of the image file picker + pixel dimension decode.
+    ImagePickResult(Result<(std::path::PathBuf, u32, u32), String>),
 }
 
 impl H7CAD {