feat(tables): column-based table detection for borderless layouts

When rect-based, line-based, and heuristic detection all fail to find
tables on a page, try building a table directly from the layout
engine's column boundaries. Handles borderless tabular layouts like
exam/reference grids where columns are defined purely by text
alignment.

Includes header-row column refinement: when a detected column
contains multiple header items, it gets split at the gap between
them to recover the correct number of columns.

Guards against false positives: requires ≥4 columns, ≤40 rows,
>50% multi-column rows, short cells (avg ≤40 chars), no prose
content, no dominant single column, and no structural elements
(≥6 rects or ≥4 lines) on the page.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/extractor/mod.rs

@@ -26,6 +26,8 @@ pub use crate::types::{ItemType, TextLine};
 pub(crate) use layout::detect_columns;
 pub use layout::group_into_lines;
 pub(crate) use layout::group_into_lines_with_thresholds;
+pub(crate) use layout::is_newspaper_layout;
+pub(crate) use layout::ColumnRegion;
 
 // ---------------------------------------------------------------------------
 // Public API