Measure the text runs of page.content() on the fallback faces #658

Context

DESIGN.md section 5: settings that change what a computation produces, such as fallback fonts, are part of its memo key. Section 14: pyropdf is compared with frozen pylopdf. #624 measures the exact boxes of words, layout, chars, and search on the fallback faces, and its Notes leave other extraction without them, so that it keeps its memo key. page.content() is that other extraction, although its docstring says that "glyph boxes are the exact boxes of the glyph outlines".

On every page of tests/assets/text/nonembedded-cjk.pdf, after a first rendering, frozen pylopdf's text runs measure the faces, and pyropdf's have no ink:

So page.content() gives no box for glyphs that page.pixmap() draws. evidence/phase1-parity does not compare page.content(), so its report does not show this.

Goal

The text runs of page.content() describe a font that a document does not embed as the fallback face that draws it, as frozen pylopdf does: its glyph boxes, the run's box, and its glyph IDs.

Scope

In: glyph boxes, run boxes, and glyph IDs of text runs; doc.fallback_fonts in the memo key of a content read with text runs; the docstring of page.content().

Out: the advances of a stretched glyph, which #656 tracks. Content reads with text_runs=False, which should keep their memo key.

Paths

crates/pyropdf-engine/src/extract/mod.rs, crates/pyropdf-engine/src/extract/model.rs, crates/pyropdf-python/src/items.rs, crates/pyropdf-python/src/document.rs, python/pyropdf/_page.py, and the tests of content items.

Acceptance

• On tests/assets/text/nonembedded-cjk.pdf, the text runs of page.content() match frozen pylopdf with its CJK extras, read after a first rendering.
• Editing doc.fallback_fonts changes the runs that the next call returns, and a call with the same faces reuses the memoized result.
• Without an installed face, the runs are as they are now, and a document whose fonts are all embedded reads no fallback face.

Notes

• Blocked by Measure the exact boxes of non-embedded CJK fonts on their fallback faces #624, which adds Hash and Eq to Fallback for a memo key and Fallback::watching.
• Frozen pylopdf fills its faces at its first rendering, so a read before any rendering gives what pyropdf gives now. Compare after one, as evidence/phase1-parity/dump.py does since Measure the exact boxes of non-embedded CJK fonts on their fallback faces #624.
• A text run has the fields that Measure the exact boxes of non-embedded CJK fonts on their fallback faces #624 keeps from a pass without faces, such as advances, so a content read cannot simply run through the faces while Keep the PDF's advances when hayro stretches a substitute glyph #656 is open: a stretched advance would move too.