fix: release page images for pages that carry no figure

[junkreef]

_release_page_resources() cleared a page's rendered image only when keep_images was
false, but keep_images is document-wide: it is true when any of
generate_page_images, generate_picture_images or generate_table_images is set. One
figure anywhere therefore pinned every page's image until _assemble_document ran, so
memory grew with the page count for the whole conversion.

The decision is now made per page. An image is kept when page images are part of the
requested output, or when the assembled page holds a FigureElement (with
generate_picture_images) or a Table (with generate_table_images). A page that has
not been assembled yet is kept, since what it holds is not yet known.

This is safe because _assemble_document maps those two element types one-to-one onto
the PictureItems and TableItems it crops, and the enrichment steps re-render through
the page backend, which keep_backend already preserves whenever they are enabled.

Verification

A 1356-page manual containing 9 figures, generate_picture_images=True,
images_scale=2.0, first 400 pages:

	peak RSS growth
before	5515 MB
after	1372 MB

Per-page growth changes from a straight line (12.4 → 13.7 MB/page as the range grows) to
a curve that flattens (8.2 → 5.7 → 3.4 MB/page). The extracted figures are byte-identical
(SHA-256) and the text and table counts are unchanged.

tests/test_page_resource_release.py is new: it exercises _page_image_has_consumer and
_release_page_resources directly across the option combinations, with a stand-in that
carries only the pipeline options, so no model is loaded. Eight of its eleven tests fail
on v2.124.0.

One behaviour note for callers: a page without a figure now returns None from
conv_res.pages[i].image unless generate_page_images is set, which is the documented
way to ask for page images.

Issue resolved by this Pull Request:
Resolves #4163

Checklist:

• Documentation has been updated, if necessary. (not necessary — no option or
  documented contract changes)
• Examples have been added, if necessary. (not necessary)
• Tests have been added, if necessary.

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