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 <[email protected]>

src/io/mod.rs

@@ -256,7 +256,7 @@ fn finite_vec3(v: &acadrust::types::Vector3) -> bool {
 }
 
 /// Returns true if the entity looks like parser garbage and should be dropped.
-fn is_entity_corrupt(e: &EntityType) -> bool {
+pub(crate) fn is_entity_corrupt(e: &EntityType) -> bool {
     use acadrust::entities::EntityType as E;
     // Reject polylines at or above this vertex count. Even valid drawings
     // rarely use this many — and parser desync produces exactly-100_000-vertex