feat(image): fetch remote rasters and clip them to their boundary

A raster image reference now resolves through one memoised path shared by
the loader and the unresolved-reference placeholder:
- http/https URLs are fetched natively (rustls, 8s timeout, 32MB cap); a
  local file is read as before. The web build never fetches. The cache is
  cleared when a new document opens.
- Online, the fetched image displays; offline or on a broken/missing
  reference, the frame + saved path text still shows. One resolver keeps the
  two in agreement without fetching twice.

The raster is also clipped to its boundary: the image draws as the
triangulated clip polygon (ear-clipped) with per-vertex UVs instead of a
full quad, so pixels appear only inside the clip. Clip-boundary Y is raster
space (top-down), so it is flipped to the image's Y-up space in both the
outline and the clip triangulation so they align.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/scene/pipeline/mod.rs

@@ -2177,7 +2177,7 @@ impl Pipeline {
                 }
                 pass.set_bind_group(1, &img.bind_group, &[]);
                 pass.set_vertex_buffer(0, img.vertex_buffer.slice(..));
-                pass.draw(0..6, 0..1);
+                pass.draw(0..img.vertex_count, 0..1);
             }
             if scissor_active {
                 pass.set_scissor_rect(0, 0, vp.width, vp.height);