M4.82: PDF export writes top-level layers as PDF layers

Create PDF layers from top-level layers (document.exportPdf createLayers, the
Save PDF dialog's General option) now writes an optional content group per
top-level layer (template layers are left out): named as the layer, off when
the layer is hidden, /PrintState /OFF when its Print option is off, locked
when it is locked, listed top first, with a print auto-state (not under a
standard). The file reopens with those layers.

The PDF writer has no optional content, so each layer is drawn as a form of
its own (no transparency group, so it draws as in place) starting with a mark;
after writing, the uses of the layers' forms are wrapped in /OC ... BDC/EMC
and the catalog gets /OCProperties (crates/pdf/src/forms.rs, through the
shared patch writer and the PDF syntax reader of encrypt.rs). PDF 1.4 and a
knockout page group write plain page content with a warning; the dialog greys
the option at 1.4. The "not written yet" warning is gone.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/pdf/src/patch.rs

@@ -48,8 +48,13 @@ impl Xref {
         Some(Self { at, entries, count, trailer: (start, end) })
     }
 
+    /// Number of entries (objects `0..count`).
+    pub fn count(&self) -> usize {
+        self.count
+    }
+
     /// Where in-use object `n` starts.
-    fn offset(&self, pdf: &[u8], n: u32) -> Option<usize> {
+    pub fn offset(&self, pdf: &[u8], n: u32) -> Option<usize> {
         let e = self.entries + 20 * usize::try_from(n).ok().filter(|n| *n < self.count)?;
         let entry = pdf.get(e..e + 20)?;
         (entry.get(17) == Some(&b'n')).then_some(())?;