v0.25.1: The page-hash job could never finish

The page-hash job could never finish

Found by watching v0.25.0 run against a real library, which is the only place it shows.

The job picks its next batch by asking for chapters that have no page fingerprints yet, and it writes a row
per page. A chapter that yields no pages — an unreadable archive, an empty one, a file that has since been
moved — therefore wrote nothing, and so was still "not looked at yet" when the next batch was chosen. Working
chapters get their rows and drop out; broken ones accumulate. The moment they are all that is left, the loop
has nothing to exhaust and spins on them, at full CPU, forever.

One such chapter in a library is enough, which on any library of real size is close to a certainty.

A chapter that produced nothing now records that it was looked at, so it drops out of the queue like any
other. That mark is inert everywhere else: it carries no fingerprint, so it can never match another page, and
it is never offered to the reader as a page to skip.

A chapter is never mostly skipped

The same first real run turned up the failure this feature is least allowed to have. Across seven thousand
fingerprinted chapters the average chapter had 1.4 pages of furniture and under 6% of all pages were
flagged — but a few hundred chapters wanted to skip a third or more of themselves, and the worst wanted 68
pages out of 88.

Those are duplicate and phantom chapters, where the same file is filed under several chapter numbers. Every
page then genuinely does recur across chapters, so the arithmetic is right and the conclusion is nonsense.
Nothing inside the rule can tell that case apart, so the chapter's own shape is the check: if more than a
third of a chapter is about to be skipped, the automatic decision is thrown away and the chapter reads
exactly as it always did. Fewer skips, which is the direction this feature is always wrong in.

A page you marked by hand is never subject to that cap. It is the one input that is not arithmetic, and the
entire point of it is that it outranks the rule.