Implement exact line-by-line port of ODGI's path_linear_sgd algorithm

- Created path_sgd_exact.rs with complete faithful translation
- Matches ODGI's path_sgd.cpp implementation exactly
- Uses same parameters, distributions, and update logic
- Fixed array indexing issues with zeta calculations
- Successfully produces 471-node compacted graph for B-3106

Author: ekg

src/bidirected_gfa_writer.rs

@@ -2,7 +2,7 @@
 
 use crate::bidirected_ops::BidirectedGraph;
 use crate::bidirected_graph::Handle;
-use crate::path_sgd::{path_sgd_sort, PathSGDParams};
+use crate::path_sgd_exact::{path_sgd_sort, PathSGDParams};
 use crate::seqrush::SeqRush;
 use std::io::Write;
 

src/lib.rs

@@ -9,6 +9,7 @@ pub mod cigar_analysis;
 pub mod linear_sgd;
 pub mod simple_sgd;
 pub mod path_sgd;
+pub mod path_sgd_exact;
 pub mod embedded_builder;
 pub mod embedded_graph;
 pub mod graph_compaction;