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>
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