Run epubcheck on generated EPUBs in tests

[willcrichton]

Running epubcheck will help us check EPUB v3 compliance in the generator.

We can also expose this as an option for the CLI, since it's possible for users to write e.g. broken links that epubcheck will pick up.

[m13v]

we run epubcheck in CI on every title and it clears about 80% of them clean. KDP still rejects about 15% of those epubcheck-clean submissions, usually remote resources, svg with embedded raster, or missing NCX fallback for older kindles. added Kindle Previewer CLI as a second pass. adds about 12 seconds per title but beats the 48 hour rejection loop. also worth running epubcheck with --mode exp to surface the accessibility warnings the default mode skips.