feat(05-02): implement visualization module with matplotlib/seaborn plots

- Add matplotlib>=3.8.0 and seaborn>=0.13.0 to dependencies
- Create visualizations.py with 3 plot functions and orchestrator
- plot_score_distribution: histogram colored by confidence tier
- plot_layer_contributions: bar chart of evidence layer coverage
- plot_tier_breakdown: pie chart of tier distribution
- Use Agg backend for headless/CLI safety
- All plots saved at 300 DPI with proper figure cleanup
- 6 tests covering file creation, edge cases, and return values

pyproject.toml

@@ -37,6 +37,8 @@ dependencies = [
     "structlog>=25.0",
     "biopython>=1.84",
     "scipy>=1.14",
+    "matplotlib>=3.8.0",
+    "seaborn>=0.13.0",
 ]
 
 [project.optional-dependencies]