Make ZIP code export lazy

- Stop importing policyengine_us_data.geography at package import time.
- Expose ZIP_CODE_DATASET through a lazy __getattr__ export instead, so importing policyengine_us_data no longer forces pandas to load.
- This breaks the modal deploy import chain that was failing in modal_app.pipeline -> modal_app.local_area -> policyengine_us_data.__init__ on the CI runner.

Author: juaristi22

policyengine_us_data/__init__.py

@@ -1,8 +1,10 @@
 from importlib import import_module
 
-from .geography import ZIP_CODE_DATASET
-
 _LAZY_EXPORTS = {
+    "ZIP_CODE_DATASET": (
+        "policyengine_us_data.geography",
+        "ZIP_CODE_DATASET",
+    ),
     "CPS_2024": (
         "policyengine_us_data.datasets.cps.cps",
         "CPS_2024",