Snapshots keep the incremental sections: undo works after a save.

pdf_save_snapshot promises the document stays unchanged, yet the
write marked every incremental section saved; undoing an earlier
step then restored nothing (Ely's PDF save and extract).

--- a/source/pdf/pdf-write.c	2026-10-06 09:03:50
+++ b/source/pdf/pdf-write.c	2026-10-06 09:03:50
@@ -2869,7 +2869,9 @@
 		pdf_sync_open_pages(ctx, doc);
 
 		// mark incremental sections as having been saved
-		doc->num_incremental_sections = 0;
+		// A snapshot keeps them, and the undo history.
+		if (!in_opts->do_snapshot)
+			doc->num_incremental_sections = 0;
 
 		pdf_end_operation(ctx, doc);
 	}