Wave-assembled from PR #3548 by @time-attack. Composition conflicts vs wave
item #3993 resolved: EmbedResult keeps both chunkless_pages_healed and the
PR's lock_skipped; KEY_FILES.md keeps the #3993 embed entry and takes this
PR's migrate-embeddings entry.
Co-Authored-By: Garry Tan <garrytan@gmail.com>