M4.53: Linked images remember their file and keep a preview; missing links ask Replace or Ignore, modified ones follow Update Links

A linked image (`ImageObject.link`, now a LinkInfo: absolute path, path relative to the document,
size, modification time, hash, page; old path strings still load) keeps a low-resolution preview
(`ImageBlob.proxy`, at most 256 px a side) that a native save writes in place of the pixels of
images only linked images show. Opening a native document reads the linked files again, looking
for each at its path, then relative to the document and by name in its folder: missing files
show their preview, modified ones are read again only with Update Links: Automatically.
`document.open` reports `missingLinks`, `modifiedLinks` and `updatedLinks`.

links.check reports each link's status, links.update reads modified files again and
links.relink points images at another file or at same-named files in a folder (each keeps its
bounds; one undo step). In the app, a document opened without its linked files asks about each
(Replace…, Ignore, Apply to All), then offers to update modified links when the preference asks.
The renderer caches previews apart from the file's pixels; the Eyedropper samples previews in
the object's pixel space.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/render/src/tests_setup.rs

@@ -1,7 +1,5 @@
 //! Document Setup options the renderer honours.
 
-use std::sync::Arc;
-
 use vectorcraft_doc::{Document, ImageBlob, ImageObject, Node, NodeKind};
 
 use super::*;
@@ -10,7 +8,7 @@ fn doc_with_red_image() -> Document {
     let mut d = Document::new(40.0, 40.0);
     let mut png = Vec::new();
     image::RgbaImage::from_pixel(4, 4, image::Rgba([255, 0, 0, 255])).write_to(&mut std::io::Cursor::new(&mut png), image::ImageFormat::Png).unwrap();
-    d.images.insert("red".into(), ImageBlob { mime: "image/png".into(), bytes: Arc::new(png) });
+    d.images.insert("red".into(), ImageBlob::new("image/png", png));
     let (id, layer) = (d.alloc_id(), d.layers[0].id);
     let im = ImageObject { key: "red".into(), width: 40, height: 40, xf: Affine::IDENTITY, link: None };
     d.insert(Some(layer), 0, Node::new(id, NodeKind::Image(im))).unwrap();