M4.47: PostScript .ai and placeholder-only .ai files say why they can't open

A PostScript file (a .ai saved in an older format or without PDF
compatibility, or an .eps, plain or with a binary header) is refused by any
name with a clear message instead of a parse error (there is no PostScript
importer yet). An .ai whose pages are only placeholder text over the editor's
private data (PieceInfo AIPrivateData) is detected and refused with advice to
save it with PDF compatibility, instead of opening the placeholder.
vectorcraft-pdf: is_postscript(), PdfError::PostScript / PlaceholderOnly.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

crates/testkit/src/pdf.rs

@@ -17,6 +17,8 @@ pub struct PdfPage {
     pub resources: String,
     /// The content stream (operators, uncompressed).
     pub content: String,
+    /// More entries of the page dictionary (`/PieceInfo << … >>`).
+    pub entries: String,
 }
 
 impl PdfPage {
@@ -90,6 +92,9 @@ pub fn pdf_with(pages: &[PdfPage], extra: &[&str], password: Option<&str>) -> Ve
         if p.rotate != 0 {
             let _ = write!(d, " /Rotate {}", p.rotate);
         }
+        if !p.entries.is_empty() {
+            let _ = write!(d, " {}", p.entries);
+        }
         d.push_str(" >>");
         objects.push(d.into_bytes());
         let num = objects.len() + 1;