feat(01-03): create DuckDB persistence layer with checkpoint-restart

- PipelineStore class for DuckDB-based storage
- save_dataframe/load_dataframe for polars and pandas
- Checkpoint system with has_checkpoint and metadata tracking
- Parquet export capability
- Context manager support

src/usher_pipeline/gene_mapping/__init__.py

@@ -0,0 +1,23 @@
+"""Gene ID mapping module.
+
+Provides gene universe definition, batch ID mapping via mygene,
+and validation gates for quality control.
+"""
+
+from usher_pipeline.gene_mapping.mapper import (
+    GeneMapper,
+    MappingResult,
+    MappingReport,
+)
+from usher_pipeline.gene_mapping.universe import (
+    fetch_protein_coding_genes,
+    GeneUniverse,
+)
+
+__all__ = [
+    "GeneMapper",
+    "MappingResult",
+    "MappingReport",
+    "fetch_protein_coding_genes",
+    "GeneUniverse",
+]