v0.25.2: Blank slices were being skipped as if they were the same page

Blank slices were being skipped as if they were the same page

The fingerprint asks, sixty-four times, whether a pixel is brighter than the one to its right. So the only
variation it can see is variation across a row. The guard meant to refuse featureless pages measured something
subtly different — the brightest and darkest pixel anywhere in the page.

Those come apart on exactly the kind of page a long-strip webtoon is full of. A slice that fades from black at
the top to white at the bottom has the widest possible range, 255, and sails through a guard asking for 8 — while
every left-to-right comparison on it is a tie. All sixty-four answers come back "no", the fingerprint is all
zeros, and every such slice in the library carries that same fingerprint. They were being matched to each other
and skipped.

This was not theoretical. On a real 42,000-chapter library the all-zero fingerprint alone was hiding 100 pages,
and in one series 59 of the 166 skipped pages were this. The guard now measures what the fingerprint actually
reads, and a page with no left-to-right variation is refused, as was always intended.

Fixing the guard is not enough by itself, because fingerprints already recorded were written by the old one and
the background job never revisits a chapter it has seen. So the same rule is applied where the matching happens:
a fingerprint in which almost every comparison was a tie is no longer accepted as evidence that two pages are the
same page. That takes effect immediately, without re-reading anything.

Some genuinely repeated near-blank separators stop being skipped as a result. They are blank slices, so this
shows up as a sliver of nothing rather than a missing panel — and fewer skips is the direction this feature is
meant to be wrong in.