fix(prepare): an `ok` record is a claim about a file, so check the file
Deleting the four 477 MB chunks left corpus.jsonl listing four paths
nothing could open, because `ok` in prepared.jsonl is a claim made once
and never revisited. The run stage would have discovered that one failed
item at a time.
The cache is gigabytes and gets pruned — deliberately now, when a
splitter's output turns out to be unsendable, and eventually just for
space — so a missing file is an ordinary state rather than a corrupt one.
corpus.jsonl now excludes them, the summary says how many and from how
many URLs, and --retry re-fetches and re-splits them even though the
URL's own outcome was never a failure.
Re-split with qpdf, the Texas act's chunks came out at 14 MB each rather
than 477 MB — 34x smaller from the same 25 pages.