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

pyproject.toml

@@ -33,6 +33,8 @@ dependencies = [
     "polars>=0.19.0",
     "pyarrow>=14.0.0",
     "pyyaml>=6.0",
+    "httpx>=0.28",
+    "structlog>=25.0",
 ]
 
 [project.optional-dependencies]