fix: don't treat bullet-marker columns as real text columns (#55)

On PDFs where every list item starts with ● at the left margin and content
at a fixed offset, histogram column detection sees the gap between marker
and content as a gutter and splits each line across two phantom "columns,"
scrambling the reading order (Anthropic's Mythos system card p.73–74).

- layout: reject gutter candidates where the smaller side is ≥80%
  standalone bullet-marker glyphs (•, ●, ○, ◦, ▪, ▫, ◆, ◇, ■, □)
- markdown/classify: add starts_with_bullet_marker helper (narrower than
  is_list_item — excludes numbered/lettered patterns like 1. and a) so
  numbered section headings stay as headings)
- markdown/convert: skip heuristic heading detection on lines that start
  with a bullet marker
- markdown/classify: strip a leading bullet wrapped in a bold/italic run
  (e.g. "**● Label:**" → "- **Label:**") — some PDFs put the marker inside
  the same bold run as the label

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

src/markdown/classify.rs

@@ -64,6 +64,22 @@ pub(crate) fn is_caption_line(text: &str) -> bool {
     false
 }
 
+/// Check if text starts with an unambiguous bullet marker (●, •, ○, ◦).
+///
+/// Narrower than [`is_list_item`]: it excludes numbered/lettered patterns
+/// like `1.` or `a)`, which legitimately appear as section headings in many
+/// documents. Used by the heading classifier to reject bullet lines without
+/// also demoting numbered headings.
+pub(crate) fn starts_with_bullet_marker(text: &str) -> bool {
+    let trimmed = text.trim_start();
+    trimmed.starts_with("• ")
+        || trimmed.starts_with("● ")
+        || trimmed.starts_with("○ ")
+        || trimmed.starts_with("◦ ")
+        || trimmed.starts_with("- ")
+        || trimmed.starts_with("* ")
+}
+
 /// Check if text looks like a list item
 pub(crate) fn is_list_item(text: &str) -> bool {
     let trimmed = text.trim_start();
@@ -115,6 +131,16 @@ pub(crate) fn format_list_item(text: &str) -> String {
         if let Some(rest) = trimmed.strip_prefix(*bullet) {
             return format!("- {}", rest.trim_start());
         }
+        // Bullet inside a leading bold/italic run (e.g. "**● Label:** rest").
+        // The run wraps both the marker and the following label because both
+        // use a bold font in the PDF.
+        for wrapper in ["**", "*"] {
+            if let Some(after_open) = trimmed.strip_prefix(wrapper) {
+                if let Some(rest) = after_open.strip_prefix(*bullet) {
+                    return format!("- {}{}", wrapper, rest.trim_start());
+                }
+            }
+        }
     }
 
     if trimmed.starts_with("- ") || trimmed.starts_with("* ") {
@@ -198,3 +224,42 @@ pub(crate) fn is_monospace_font(font_name: &str) -> bool {
 
     patterns.iter().any(|p| lower.contains(p))
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn format_list_item_plain_bullet() {
+        assert_eq!(format_list_item("● Item"), "- Item");
+        assert_eq!(format_list_item("• Item"), "- Item");
+    }
+
+    #[test]
+    fn format_list_item_bullet_inside_bold() {
+        // PDF that uses bold font for both the marker and the label produces
+        // a single bold run like "**● Label:** rest"; the bullet must still
+        // be stripped and the bold wrapper preserved on the label.
+        assert_eq!(
+            format_list_item("**● Fraud: Willing cooperation;**"),
+            "- **Fraud: Willing cooperation;**"
+        );
+        assert_eq!(
+            format_list_item("**● Label:** rest of line"),
+            "- **Label:** rest of line"
+        );
+        assert_eq!(format_list_item("*● Italic:* rest"), "- *Italic:* rest");
+    }
+
+    #[test]
+    fn format_list_item_already_dash() {
+        assert_eq!(format_list_item("- existing"), "- existing");
+    }
+
+    #[test]
+    fn is_list_item_with_bullet_space() {
+        assert!(is_list_item("● Item"));
+        assert!(is_list_item("• Item"));
+        assert!(is_list_item("- Item"));
+    }
+}