perf: cache hatch/wipeout/image/mesh models per geometry epoch (F+G+H)

Add Arc<Vec<...>>-backed epoch caches for hatches, wipeouts, images, and
meshes — the same pattern used for wires. build_primitive() now returns
an O(1) Arc refcount bump on navigation frames instead of rebuilding
every collection from scratch.

Also wrap ImageModel.pixels in Arc<Vec<u8>> so cloning an ImageModel is
O(1) regardless of image resolution.

Navigation frames no longer copy pixel data, tessellated hatch boundaries,
or mesh geometry — all per-frame CPU work on unchanged geometry is now
eliminated.

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

src/scene/pipeline/image_gpu.rs

@@ -87,7 +87,7 @@ impl ImageGpu {
         });
         queue.write_texture(
             texture.as_image_copy(),
-            &model.pixels,
+            &model.pixels[..],
             wgpu::TexelCopyBufferLayout {
                 offset: 0,
                 bytes_per_row: Some(4 * model.width),