feat(01-02): create mapping validation gates with tests

- Add MappingValidator with configurable success rate thresholds (min_success_rate, warn_threshold)
- Add validate_gene_universe for gene count, format, and duplicate checks
- Add save_unmapped_report for manual review output
- Implement 15 comprehensive tests with mocked mygene responses (no real API calls)
- Tests cover: successful mapping, notfound handling, uniprot list parsing, batching, validation gates, universe validation

src/usher_pipeline/gene_mapping/__init__.py

@@ -13,6 +13,11 @@ from usher_pipeline.gene_mapping.universe import (
     fetch_protein_coding_genes,
     GeneUniverse,
 )
+from usher_pipeline.gene_mapping.validator import (
+    MappingValidator,
+    ValidationResult,
+    validate_gene_universe,
+)
 
 __all__ = [
     "GeneMapper",
@@ -20,4 +25,7 @@ __all__ = [
     "MappingReport",
     "fetch_protein_coding_genes",
     "GeneUniverse",
+    "MappingValidator",
+    "ValidationResult",
+    "validate_gene_universe",
 ]