feat(tables): extract tables from tagged PDF structure tree (#17)

When a PDF has a well-formed structure tree with /Table > /TR > /TD|TH
elements linked to MCIDs, build tables directly from the semantic
hierarchy. Runs as highest-priority detection (step 0) before rect-based,
line-based, and heuristic strategies.

- Add StructTree::extract_tables() to walk the tree and collect table
  descriptors with row/cell/MCID info
- Add detect_tables_from_struct_tree() to match MCIDs to TextItems
- Reject tables with <30% MCID cell coverage (stale structure trees)
- Update 2013-app2 snapshot (struct-tree gives valid but different
  column ordering)

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

src/tables/mod.rs

@@ -5,6 +5,7 @@
 mod detect_heuristic;
 mod detect_lines;
 mod detect_rects;
+mod detect_struct;
 mod financial;
 mod format;
 mod grid;
@@ -13,6 +14,7 @@ pub use detect_heuristic::detect_tables;
 pub use detect_lines::detect_tables_from_lines;
 pub(crate) use detect_rects::cluster_rects;
 pub use detect_rects::{detect_tables_from_rects, RectHintRegion};
+pub use detect_struct::detect_tables_from_struct_tree;
 pub use format::table_to_markdown;
 
 use crate::types::TextItem;