XFDF: gracefully ignore invalid Unicode annotation colours (#48)

Use checked string slices when reading annotation colour components. Cover colour and interior-colour imports with a synthetic XFDF regression.

Co-authored-by: storybored379 <storybored379@users.noreply.github.com>
Co-authored-by: OpenAI Codex <noreply@openai.com>

crates/xfdf/src/lib.rs

@@ -140,7 +140,7 @@ fn parse_hex(s: &str) -> Option<[f64; 3]> {
     if h.len() != 6 {
         return None;
     }
-    let p = |i: usize| u8::from_str_radix(&h[i..i + 2], 16).ok().map(|v| f64::from(v) / 255.0);
+    let p = |i: usize| h.get(i..i + 2).and_then(|s| u8::from_str_radix(s, 16).ok()).map(|v| f64::from(v) / 255.0);
     Some([p(0)?, p(2)?, p(4)?])
 }
 

crates/xfdf/src/tests.rs

@@ -204,3 +204,15 @@ fn data_files_merge_into_a_spreadsheet() {
     assert!(lines[3].starts_with("\"Grace \"\"Amazing\"\" Hopper, RADM\",Off,"), "{csv}");
     assert_eq!(data_values(b"not data"), Err(DataError::UnknownFormat));
 }
+
+#[test]
+fn unicode_annotation_colours_are_ignored_without_panicking() {
+    for attr in ["color", "interior-color"] {
+        let mut doc = blank();
+        let xml = format!(
+            r##"<xfdf xmlns="http://ns.adobe.com/xfdf/"><annots><square page="0" rect="10,10,50,50" name="unicode-colour" {attr}="#€€" /></annots></xfdf>"##
+        );
+        let report = import(&mut doc, xml.as_bytes()).unwrap();
+        assert_eq!(report.comments, 1);
+    }
+}