M6: reject unreadable PDF trailer history links (#1670)

Reject a non-null Prev entry that cannot be read as an offset instead of silently treating it as the end of the guarded trailer chain. Keep absent/null end markers and readable numeric offsets unchanged.

Reproduced CodeRabbit discussion_r4366245854 with an independent probe and a failing native regression before the fix. The synthetic regression covers seven unreadable value types plus absent/null and integer/real history controls.

Validation: all 9 cargo xtask ci gates passed (3063 tests passed, 6 skipped; 16 WASM-safe crates). One unrelated GPU test was labelled leaky by nextest in that run and passed cleanly in an isolated rerun. Independent checks: 17 original supported PDF variants still pass qpdf, logical preservation and Poppler pixels; 6 original refusal cases remain refused. All 11 additional Prev controls pass (7 refused, 4 accepted).

crates/engine/src/cmd/pdflayers.rs

@@ -263,6 +263,28 @@ pub(crate) mod tests {
         s.execute("file.exportPdf", &json!({})).unwrap();
     }
 
+    #[test]
+    fn unreadable_non_null_prev_entries_refuse_layer_rewrites() {
+        for prev in ["7 0 R", "-1", "false", "<<>>", "[]", "/Unknown", "(unknown)"] {
+            let bytes = two_layer_pdf_options(0, "", &format!("/Prev {prev}"));
+            assert_eq!(designcraft_render::pdf_layers(&bytes).len(), 2, "reader control: {prev}");
+            assert!(designcraft_render::pdf_hide_layers(&bytes, &["Red".into()]).is_none(), "must not truncate unknown history: {prev}");
+        }
+        for trailer in ["", "/Prev null"] {
+            let bytes = two_layer_pdf_options(0, "", trailer);
+            assert!(designcraft_render::pdf_hide_layers(&bytes, &["Red".into()]).is_some(), "end-of-chain control: {trailer}");
+        }
+        let mut revised = two_layer_pdf();
+        let prev =
+            std::str::from_utf8(&revised).unwrap().rsplit_once("startxref").unwrap().1.split_whitespace().next().unwrap().parse::<usize>().unwrap();
+        add_pdf_revision(&mut revised);
+        let real_prev = std::str::from_utf8(&revised).unwrap().replace(&format!("/Prev {prev}>>"), &format!("/Prev {prev}.0>>"));
+        assert!(real_prev.contains(&format!("/Prev {prev}.0>>")));
+        for bytes in [revised, real_prev.into_bytes()] {
+            assert!(designcraft_render::pdf_hide_layers(&bytes, &["Red".into()]).is_some(), "readable integer/real history must still work");
+        }
+    }
+
     #[test]
     fn fake_null_trailer_cannot_bypass_hybrid_rejection() {
         for real_guard in ["", "/XRefStm 0"] {

crates/render/src/pdf_layers.rs

@@ -176,6 +176,9 @@ pub fn pdf_hide_layers(bytes: &[u8], hidden: &[String]) -> Option<Vec<u8>> {
             return None;
         }
         offset = t.get::<usize>(b"Prev");
+        if offset.is_none() && has_non_null_entry(&t, b"Prev") {
+            return None;
+        }
     }
     if offset.is_some() {
         return None;