[Velli20]

Summary

In the web stack (pdfium-web), checkbox, radio and push-button widgets used to show as native browser controls (blue checkboxes, push buttons showing only their field label). PDFium draws each widget's /AP appearance stream instead. This PR paints those appearances on the page canvas, matching PDFium.

• Core (pdf-annotation-core):
  • AppearanceDictionary keeps the resolved /AP /N and /D entries. They are unparsed and not serialized.
  • WidgetControl.painted marks button widgets that have an appearance.
  • AnnotationOverlay::painted_appearances(page, pressed) returns each displayed widget's appearance under its live state: the on-state or Off, /AS for push buttons, and /D while pressed.
• Renderer: PdfRenderer::render_annotation_appearances maps each form's transformed /BBox onto /Rect (ISO 32000 §12.5.5). It draws through the existing content-stream path after the page content, without recording selectable text. An appearance stream that can't be read is skipped.
• pdf-web:
  • WebPageRenderer::render paints the appearances. The conformance --renderer web harness goes through this path too.
  • Painted controls are transparent (native-painted), so they still take clicks and keyboard focus.
  • Non-button fields get PDFium's form highlight, rgb(221 228 255) at alpha 100/255, through --native-form-highlight.
  • A new optional onPress callback reports a painted widget being held down, so the host can draw its down appearance.
• Example host: WebDocument::repaint and WebDocument::press re-render a page with its stored viewport, after edits and while a widget is held down.

Testing

• cargo clippy with -D warnings on the changed crates, natively and for wasm32-unknown-unknown.
• cargo fmt --check, cargo test on the changed crates, and cargo xtask annotation-types --check.
• Rendered action_hide_show_form.pdf (from the PDFium corpus) through the new appearance pass on Skia. The checkbox and radio glyphs, the button logos and the captions match PDFium.
• Viewed in the browser (cargo xtask web-canvas). The glyph and highlight colours match PDFium's render: glyphs (87,89,99), field background (242,244,254).
• Not checked: the pressed (/D) appearance has only been built, not seen in a browser or rendered through Skia.

Not run locally: clippy with --all-features, which needs a Skia build with GL/PDF features that isn't cached here, and the tsc type check, which isn't installed. CI runs both.

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