Part of #868: PDFs saved with the reference app's editing data open from it, as .ai files do

A PDF saved with the reference app's editing data keeps that data in
streams named AIPDFPrivateData1, AIPDFPrivateData2, ..., where a .ai
file uses AIPrivateData1, AIPrivateData2, .... illustrator_data read
only the .ai names, so such a PDF opened like any other PDF: plain
groups opened ungrouped, and hidden objects and guides were missing. The
import gave no warning that the PDF's editing data went unread.

illustrator_data now reads the AIPDFPrivateData streams when the file
has no AIPrivateData1, and the PDF opens from its editing data as a .ai
file does. editingData: false still opens it like any other PDF.
has_private_data still looks for AIPrivateData keys only.

layered_ai also returns whether it read the layers. When it did, art
reaching past a PDF's page opens whole, and through_editing_data drops
the PDF importer's note that such art was clipped (PAST_PAGE_NOTE, now
public) as it drops the off-artboard note. When it didn't, both notes
stay. On main, a .ai whose editing data can't be decoded or has no
layers opened from its PDF part without the off-artboard note.

testkit::ai::pdf writes the .ai file of testkit::ai::ai with the PDF
key names. The editing-data tests open it as a PDF, and the import fuzz
test opens the damaged sample as a PDF too.

Docs: the document.open params, docs/mcp.md,
docs/file-format-parity.md, docs/gaps.md (G9: the export direction of
#868 remains), one evidence line in docs/target-app-parity.md, and #868
moved from layer structure to editing data in the File formats row of
docs/target-app-parity.md and in docs/roadmap.md.

docs/mcp.md

@@ -496,7 +496,8 @@ pattern fills, placed files or anything else the reader doesn't read on a layer
 different from its page, opens as before: an EPS as its printed page, a `.ai` file from its PDF content (where plain
 groups open ungrouped), with a warning saying why. Layers look different when they draw an object the page doesn't (or
 miss one it draws), and, for an EPS whose page is the box of its art, when they print art outside it. With
-`textAs: "outlines"` a `.ai` file whose type shows opens from its PDF content, which has the type's outlines.
+`textAs: "outlines"` a `.ai` file whose type shows opens from its PDF content, which has the type's outlines. A PDF
+saved with Illustrator's editing data holds it in `AIPDFPrivateData` streams and opens from it as a `.ai` file does.
 
 The document then has what the file's page doesn't print: hidden objects and layers, layers that don't print, guides and
 the art outside the artboards, as the app that wrote the file shows them. Exports and `artboard.fitToArt` leave out the