feat(pdf): share one image across the pages that place it

[andiwand]

🤖 Generated with Claude Code

Stacked on #750, which is stacked on #749.

The pdf view inlined every image as a data url and ignored HtmlConfig::embed_images — every other view honours it. A logo on 100 pages was base64'd 100 times.

Measured on the private pdf corpus, splitting duplication by where it occurs:

file	within one page	across pages
Rockrider_e-Actv500_Allgemein_und_Garantie	0.0M	29.7M of 49.5M
Rockrider_e-Actv500_Gebrauchsinformationen	1.6M	14.3M of 28.0M
Laserentfernungsmesser_PLEM50_D5	0.0M	7.4M (10 images, 279 placements)
Normstahl_UltraS_Garagentorantrieb_Hans	0.0M	2.0M

It is almost entirely across pages, so collapsing repeats inside one page with <defs>/<use> would recover close to nothing. Sharing a resource does.

How

ImageRegistry keys on the image XObject. The parser already memoizes those per document, so the address identifies a repeat and no digest over the bytes is needed — the resource is named by a plain counter, and the extension comes from file_type_by_mimetype / file_extension_by_file_type rather than a new table.

The resource goes through config.resource_locator, so:

• embed_images on (the default) — a data url, exactly as today
• embed_images off — image1.jpg written once, linked from every page that places it

An inline (BI) image and a stencil get an entry each: a stencil's bytes are built from the fill colour in effect, so two placements in different colours are genuinely different images.

Verification

• Two new tests: one image on two pages is one resource referenced twice with nothing inlined; with embedding on, both placements stay inline and no location is handed out.
• Default-config output over the 11-file corpus is byte-identical — same totals, same per-page sizes, same node counts.
• Full suite 1228 passed, 0 failed, 6 pre-existing skips.

Note for the apps

The saving only lands when the host sets embed_images = false. Before this PR that flag did nothing for pdf, so nothing changes until an app opts in.