feat: insert another design as a linked component (#596)

component.insert_design adds a component holding another design's solid bodies, linked to its file (stored relative to the assembly once saved). component.update_links reads the linked designs again, component.break_link keeps the bodies and forgets the file, and component.links lists them. Links are saved under format 16; an older build keeps the bodies and drops the link.

Fixes #533

crates/engine/src/cmd/file.rs

@@ -190,6 +190,26 @@ fn write_file(s: &mut Session, path: &str, format: &str, compression: &str, prev
         "compact" => solidcraft_io::DesignCompression::Compact,
         _ => return Err(bad("file.save", "compression must be fast or compact")),
     };
+    // Linked designs are kept relative to the folder the design is saved in.
+    let links = super::component_link::rebased(&s.doc, s.path.as_deref(), path);
+    let old_links = (links != s.doc.links).then(|| std::mem::replace(&mut s.doc_mut().links, links));
+    let r = write_bytes(s, path, format, compression, preview, cache);
+    if r.is_err()
+        && let Some(old) = old_links
+    {
+        s.doc_mut().links = old;
+    }
+    r
+}
+
+fn write_bytes(
+    s: &mut Session,
+    path: &str,
+    format: &str,
+    compression: solidcraft_io::DesignCompression,
+    preview: bool,
+    cache: bool,
+) -> Result<Value> {
     let bytes = match format {
         "container" => s.design_container(compression, preview, cache)?,
         "json" => solidcraft_io::write_design(&s.design()),