feat: add WatermanSmithBeyer algorithm

[MavenRain]

Summary

Closes #7.

Adds the WatermanSmithBeyer algorithm, a global alignment algorithm that uses
affine gap penalties and considers all possible gap lengths (O(n^3)).

Design

• Uses ExtendedGapScoring (gap open + gap extend) with cost function w(k) = gap_open + (k-1) * gap_extend
• Stores optimal gap step sizes in the extra pointer matrices from
  AlignmentData::new_gotoh(), avoiding any changes to AlignmentData or
  Matrix2D
• Custom WsbAligner traceback iterator handles variable-length gaps (pushes
  multiple characters per gap step)
• Follows the same GlobalAlignmentMatrix trait pattern as NeedlemanWunsch

Files

• src/alignment/edit/waterman_smith_beyer.rs (new) -- scoring matrix
  computation
• src/alignment/global_base.rs -- added WatermanSmithBeyer variant and
  WsbAligner traceback
• src/alignment/edit/mod.rs, src/lib.rs -- module registration and
  re-export
• tests/wsb_test.rs (new) -- 9 tests

Test plan

• 33 tests pass (24 existing + 9 new)
• cargo clippy clean
• cargo fmt clean