feat(02-01): add gnomAD constraint data models and download module

- Create evidence layer package structure
- Define ConstraintRecord Pydantic model with NULL preservation
- Implement streaming download with httpx and tenacity retry
- Add lazy TSV parser with column name variant handling
- Add httpx and structlog dependencies

src/usher_pipeline/evidence/__init__.py

@@ -0,0 +1 @@
+"""Evidence layer package for external data sources."""