fix: detect hierarchical-indent TOCs as tables

Validation 1 (≥25% of rows have first col) and validation 9 (paragraph
content) were rejecting TOC pages where top-level chapters indent at the
leftmost X but subsections cascade further right. The leftmost column ends
up sparse (only chapter rows land there) and most cells are empty, but the
structure is still an unambiguous TOC.

Skip both validations when cells form a TOC pattern AND the table is narrow
(≤5 cols). The width cap preserves existing handling of wide multi-column
TOCs (e.g. back-of-book 2-up indices) where format_toc_as_list would mash
adjacent visual entries together.

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

src/tables/detect_heuristic.rs

@@ -581,8 +581,14 @@ fn detect_table_in_region(items: &[(usize, &TextItem)], mode: TableDetectionMode
     // Validation 1: some rows should have content in first column.
     // Use a lower threshold (25%) for tables with wrapped cells where
     // continuation lines leave the first column empty.
+    // Skip when cells form a narrow TOC pattern: hierarchical entries indented
+    // across multiple X levels leave the leftmost column sparse (only top-level
+    // chapters land there) but the structure is still a valid TOC. Narrow only
+    // (<=5 cols) — wide multi-column TOCs (e.g. 2-up indices) would render
+    // poorly through format_toc_as_list, which assumes one entry per row.
     let rows_with_first_col = cells.iter().filter(|row| !row[0].is_empty()).count();
-    if rows_with_first_col < rows.len() / 4 {
+    let is_narrow_toc = columns.len() <= 5 && is_table_of_contents(&cells);
+    if rows_with_first_col < rows.len() / 4 && !is_narrow_toc {
         log::debug!(
             "  validation 1 fail: {}/{} rows have first col",
             rows_with_first_col,
@@ -653,8 +659,12 @@ fn detect_table_in_region(items: &[(usize, &TextItem)], mode: TableDetectionMode
         return None;
     }
 
-    // Validation 8: Reject paragraph-like content falsely detected as tables
-    if is_paragraph_content(&cells) {
+    // Validation 8: Reject paragraph-like content falsely detected as tables.
+    // TOC pages with deep indentation (top-level chapters in col 0, subsections
+    // in cols 1-3, page numbers in last col) leave most cells empty and trip
+    // the paragraph heuristic; TOC shape is a safer signal here. Narrow only
+    // — see narrow-TOC rationale at validation 1.
+    if is_paragraph_content(&cells) && !is_narrow_toc {
         log::debug!("  validation 9 fail: paragraph content");
         return None;
     }
@@ -1601,6 +1611,62 @@ mod tests {
         );
     }
 
+    #[test]
+    fn is_table_of_contents_accepts_hierarchical_indented_toc() {
+        // Mythos system card pages 4-5: top-level chapters indent at col 0,
+        // subsections at cols 1-2, leaving col 0 mostly empty (only ~10% of
+        // rows). Validation 1 was rejecting these even though the structure
+        // is unambiguously a TOC.
+        let cells = vec![
+            vec!["Abstract".to_string(), String::new(), "3".to_string()],
+            vec![
+                "1 Introduction".to_string(),
+                String::new(),
+                "10".to_string(),
+            ],
+            vec![
+                String::new(),
+                "1.1 Model training".to_string(),
+                "11".to_string(),
+            ],
+            vec![
+                String::new(),
+                "1.1.1 Training data".to_string(),
+                "11".to_string(),
+            ],
+            vec![
+                String::new(),
+                "1.1.2 Crowd workers".to_string(),
+                "12".to_string(),
+            ],
+            vec![
+                String::new(),
+                "1.2 Release decision".to_string(),
+                "13".to_string(),
+            ],
+            vec![
+                "2 RSP evaluations".to_string(),
+                String::new(),
+                "16".to_string(),
+            ],
+            vec![
+                String::new(),
+                "2.1 RSP risk assessment".to_string(),
+                "16".to_string(),
+            ],
+            vec![String::new(), "2.1.1 Context".to_string(), "16".to_string()],
+            vec![
+                String::new(),
+                "2.2 CB evaluations".to_string(),
+                "20".to_string(),
+            ],
+        ];
+        assert!(
+            is_table_of_contents(&cells),
+            "hierarchical TOC with sparse col 0 should still be detected"
+        );
+    }
+
     #[test]
     fn is_table_of_contents_rejects_dotless_toc() {
         // Tabular TOC without leader dots: first column starts with dotted