feat(pdf): paint annotation appearance streams

[andiwand]

🤖 Generated with Claude Code

Stacked on #703.

A filled form rendered as empty boxes. What a form field displays is its widget
annotation's appearance stream, and annotations only ever reached the HTML
layer as raw dictionaries, for links — nothing painted them.

How

The parser resolves /AP /N into a form XObject, following /AS when /N is
a dictionary of states (a check box's /Off and /Yes), and stores it on the
Annotation along with the matrix that fits its /Matrix-transformed /BBox
onto the annotation's /Rect — 12.5.5's algorithm. extract_annotation then
runs that form the way Do runs any other: fit matrix, the form's own
/Matrix, the /BBox clip. So an appearance yields ordinary page elements,
painted after the page content, and the text one draws is selectable like any
other text.

Not painted: annotations flagged Hidden or NoView (/F), and popups, whose
appearance shows only while the viewer has them open. AcroForm interactivity
stays out of scope — the appearance is whatever the writer left in the file,
never regenerated from /V and /DA.

Test

• Three unit tests over the placement: the fit onto /Rect, the form
  /Matrix concatenating onto it, and an annotation with nothing to paint.
• Full suite green. Two reference-output files change and both now match a
  ghostscript render of the same file: phd-agreement.pdf shows every filled
  value and the selected radio button, VZA_Ankauf_Kunde.pdf its filled values
  and its /Square markup.

Note

/Rect corners are routinely indirect, and one deep_resolve reaches only the
array — its elements stay references, which as_reals cannot read. The rect is
resolved element by element instead. That is why VZA_Ankauf_Kunde.pdf threw
bad any cast the moment annotations started being parsed.

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