Skip signed hex codes in Tagged Text <0x…> tags

Found while testing crates/textimport/src/tagged.rs:~190 (the `n.starts_with("0x")` branch of import: `u32::from_str_radix(&n[2..], 16)`)

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

crates/textimport/src/tagged.rs

@@ -176,7 +176,10 @@ pub fn import(bytes: &[u8]) -> Result<Imported, ImportError> {
                 let (name, value) = tag.split_once(':').map_or((tag.as_str(), ""), |(a, b)| (a, b));
                 match name {
                     n if n.starts_with("0x") => {
-                        if let Ok(u) = u32::from_str_radix(&n[2..], 16)
+                        // `from_str_radix` accepts a leading '+', so check the digits first.
+                        if let Some(digits) = n.get(2..)
+                            && digits.bytes().all(|b| b.is_ascii_hexdigit())
+                            && let Ok(u) = u32::from_str_radix(digits, 16)
                             && let Some(ch) = units.push(u)
                         {
                             push(&mut paras, ch, &fmt);
@@ -278,6 +281,16 @@ mod tests {
         assert_eq!(import("<ASCII-WIN>\r\n<ParaStyle:Body>A<0xD83D>B<0xDE00>C".as_bytes()).unwrap().story.text, "ABC");
     }
 
+    /// `from_str_radix` accepts a leading `+`, so `<0x+41>` inserted an 'A'. A signed or empty
+    /// code is not a code point tag and is skipped.
+    #[test]
+    fn a_signed_hex_code_tag_is_skipped() {
+        for t in ["<0x+41>", "<0x-41>", "<0x>", "<0xZZ>"] {
+            let src = format!("<ASCII-WIN>\r\n<ParaStyle:Body>A{t}B");
+            assert_eq!(import(src.as_bytes()).unwrap().story.text, "AB", "{t}");
+        }
+    }
+
     /// A size does not have to be non-finite to be nonsense: a file can hold `1e300`, `0` or a
     /// negative, and tagged text took all three and set them on the text (#1593). They are now
     /// dropped, as RTF and DOCX already dropped them, so the text keeps its style's size.