feat: detect tables from PDF line path operators (m/l/S)

Many IRS forms and government PDFs draw table gridlines using path
operators (m/l/S) instead of rectangle (re) operators. This adds
line-based table detection to capture these tables.

- Add PdfLine type for line segments from path operators
- Capture m/l/h/S/s/B/b/f/n path operators in content_stream.rs
- Thread Vec<PdfLine> through extraction pipeline
- New detect_lines.rs: classify lines, snap to grid, validate and
  assign items with extensive false-positive filters
- Integrate in markdown pipeline: rects first, then lines as fallback

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/tables/mod.rs

@@ -3,12 +3,14 @@
 //! Detects tabular data in PDF text items and converts to markdown tables.
 
 mod detect_heuristic;
+mod detect_lines;
 mod detect_rects;
 mod financial;
 mod format;
 mod grid;
 
 pub use detect_heuristic::detect_tables;
+pub use detect_lines::detect_tables_from_lines;
 pub use detect_rects::{detect_tables_from_rects, RectHintRegion};
 pub use format::table_to_markdown;