ocda: the images an artefact carries, and the ones it only describes

The document markup sample collection builds byte identically from
either artefact: 1457 files, no differences, from markup, from the 35
.ssp files, and from the 35 .ocda.db files.

A .ocda.db carries its images as blobs. dbReadFiles() reads them and
they are written where the output writers look for images: a
directory of this run's making, removed when the run ends, not the
pod beside the artefact, (which could be a working published tree). The
document's source path moves with it, image_dir_path being reached from
the document's own file rather than from the pod.

Each blob is checked against the sha256 the database recorded beside it.
The two were written together, so a mismatch means the file is damaged.
(the point of having recorded the digest).

A .ssp only describes its images, so those are the ones in the pod it
sits in, and they are checked against the digests it recorded.

That asymmetry arises from of what the two artefacts are: the ocda.db is
self-sufficient and can prove it is looking at the right image, the .ssp
describes a pod it sits alongside.

Found while wiring it: setting the pod directory alone was not enough,
since image_dir_path is derived from the source file's path
(../../image from media/text/<lang>/), so the source path has to be
moved with it.

(assisted by Claude-Code)

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