fix(core): UTF-8 char boundary panic in find_content_position (#16) (#24)

`search_from = abs_pos + 1` landed mid-char when a rejected match
started on a multi-byte UTF-8 character, panicking on the next
`markdown[search_from..]` slice. Advance by `needle.len()` instead —
always a valid char boundary, and skips the whole rejected match
instead of re-scanning inside it.

Repro: webclaw https://bruler.ru/about_brand -f json
Before: panic "byte index 782 is not a char boundary; it is inside 'Ч'"
After:  extracts 2.3KB of clean Cyrillic markdown with 7 sections

Two regression tests cover multi-byte rejected matches and
all-rejected cycles in Cyrillic text.

Closes #16

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/webclaw-core/src/extractor.rs

@@ -684,7 +684,11 @@ fn find_content_position(markdown: &str, needle: &str) -> Option<usize> {
         if !is_inside_image_syntax(markdown, abs_pos) {
             return Some(abs_pos);
         }
-        search_from = abs_pos + 1;
+        // Advance past the rejected match. `abs_pos + needle.len()` is always a
+        // valid UTF-8 char boundary (end of the matched substring); `abs_pos + 1`
+        // is not, and panics on the next slice when the match starts on a
+        // multi-byte char (Cyrillic, CJK, accented Latin, emoji). See issue #16.
+        search_from = abs_pos + needle.len();
     }
     None
 }
@@ -859,6 +863,36 @@ mod tests {
         Html::parse_document(html)
     }
 
+    /// Regression: issue #16 — `find_content_position` used to advance
+    /// `search_from` by 1 byte after an image-syntax rejection, which landed
+    /// mid-char on multi-byte UTF-8 input (Cyrillic, CJK, accented Latin, emoji)
+    /// and panicked on the next `markdown[search_from..]` slice.
+    #[test]
+    fn find_content_position_does_not_panic_on_multibyte_rejected_match() {
+        // `needle` appears first inside image syntax (must be rejected), then
+        // again as plain content after a block of Cyrillic prose. The bump
+        // from the rejected match used to land inside 'Ч'.
+        let markdown =
+            "![alt needle text](/img.png) Наша история Brûler d'Amour. needle text appears here.";
+        let pos = find_content_position(markdown, "needle text");
+        assert!(pos.is_some(), "second occurrence should be found");
+        assert!(
+            markdown.is_char_boundary(pos.unwrap()),
+            "returned offset must be a char boundary"
+        );
+    }
+
+    #[test]
+    fn find_content_position_survives_all_rejected_in_cyrillic() {
+        // Every occurrence of `needle` sits inside image syntax, so the
+        // function must walk the whole string rejecting each one. With the
+        // `+1` bug this panicked the first time `search_from` crossed a
+        // 2-byte char. With the fix it should return None cleanly.
+        let markdown =
+            "Наша история ![foo needle bar](a.png) Ещё текст ![needle](b.png) Конец 'Ч'";
+        assert_eq!(find_content_position(markdown, "needle"), None);
+    }
+
     /// Helper: extract with default options (backward-compatible).
     fn extract_default(doc: &Html, base_url: Option<&Url>) -> Content {
         extract_content(doc, base_url, &ExtractionOptions::default())