feat(ja.soraraw): split stacked chapter images into pages

[kinboy56]

Some chapters here ship as one image with every page stacked on top of each other, up to 49152
pixels tall. Version 2 refused those with an error. This splits them instead.

• get_page_list measures the image header and expands a stacked image into one page per stacked
  page. The extension doesn't say whether the file is jpeg or webp, so the magic bytes decide.
• PageImageProcessor cuts the slice each page asks for.
• A count past 64 pages is read as a bad header rather than an image that deep, and is left whole.
• Ordinary chapters take no extra request and never reach the processor.

minAppVersion is now 0.8.5 and the source version 3. It depends on two fixes that
landed after v0.8.4:

• AidokuRunner e44d774: drawing into a canvas shorter than the source image.
• Aidoku 3fd2913: pages sharing a url and differing only by their PageContext all rendered the
  same image.

Tell me if the next release will be numbered otherwise and I will change it. 3fd2913 is also what
let the ?slice=N workaround the earlier revision carried go, so the image is now fetched once per
chapter rather than once per page.

Device testing covered the ?slice=N revision (iPhone 15, iOS 26.1).
The current one needs a run on a nightly carrying 3fd2913

[kkantan]

since this depends on a fix that is not in a public app version, we should wait to merge until the next app release. the minimum app version should be set to whatever the next version release is.