Two additions the epub writer needed and could not have without.

ocda.images: an image's bytes, from whichever input form the document came
from. The model names images, with digest, byte size and pixel size, and
does not hold them - which is right, an abstraction is not a bundle of
assets - so something has to know where they are, and the answer depends
on the input form rather than on the output. Hence the library.

  .ocda.db   the files table, role 'image', by name
  .sisupod   the archive entry whose path ends in /image/<name>
  .zip       the same
  .ssp       beside it: a pod puts the abstraction in media/abstraction/
             and its images in media/image/, so the path is documented
             rather than guessed

canGiveImages says whether a source can answer at all, because a writer
has to know before it emits an <img>: a manifest item whose file is absent
and an <img> whose file is not in the manifest are both epubcheck errors.
A .ssp outside a pod has nowhere to look, and that is absence rather than
error - the document is still readable.

Reading a blob needed sqlite3_column_blob rather than _column_text: an
image read as text stops at the first zero byte, and a png's ninth byte
is zero.

And two more model fields: segment_epub and segment_epub_is. The epub
segments at heading levels 1 to 4 where the html segments at level 4
alone, so the two are different files and the abstraction records both.
Nine of the 36 documents have images; the four-way agreement still holds
at 36 of 36 each way.

(assisted by Claude-Code)