arxiv pdf: download to temp file then move, so a failed download no
longer poisons re-entry with a corrupt partial file. e-print tar:
--max-size is not a GNU tar option and silently did nothing; extract
first then audit with du, failing if >50M.
Add the ledger schema, class router, lint codes, ingest/build
pipeline, and three work-tree examples: align derivation,
superfig delegation, and supertensor delegation.
dela