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 <[email protected]>

src/scene/mod.rs

@@ -235,6 +235,10 @@ pub struct OpenTimings {
 /// Build hatch / image / mesh caches from a document without needing `&mut Scene`.
 /// Intended to run on a background thread during file load.
 pub fn build_derived_caches(doc: &CadDocument) -> DerivedCaches {
+    // A new drawing must not inherit the previous one's resolved images — drop
+    // the memoised set so each reference re-reads / re-fetches once here (and
+    // stays cached across this document's later cache rebuilds).
+    crate::scene::model::image_model::clear_image_cache();
     // model-space block handle (same logic as Scene::model_space_block_handle)
     let model_block = doc
         .objects