perf(io,scene): drop redundant purge walk, fuse world-offset scan

Phase 1.1: resolve_xrefs now purges corrupt entities inline as it
merges each xref and returns the dropped count, removing the second
full-document purge_corrupt_entities walk in the FileOpened handler.
Every xref-bearing open saves one O(N) entities() pass.

Phase 2.4: split compute_world_offset into offset_prep / offset_centroid
/ world_offset_from_centers; centroid accumulation now rides the existing
cache-handle walk in build_derived_caches. Two O(N) entity walks collapse
into one, with identical MSPACE-membership filtering preserved.

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

src/app/commands.rs

@@ -641,7 +641,7 @@ impl OpenCADStudio {
                 // Reload all xrefs for the current drawing.
                 if let Some(path) = &self.tabs[i].current_path.clone() {
                     if let Some(base_dir) = path.parent() {
-                        let infos = crate::io::xref::resolve_xrefs(
+                        let (infos, _dropped) = crate::io::xref::resolve_xrefs(
                             &mut self.tabs[i].scene.document,
                             base_dir,
                         );