Expose dataset-free baseline parameter export

[anth-volk]

Fixes #111
Refs PolicyEngine/policyengine-uk-chat#141

Summary:

• Adds package-level get_baseline_params(year=...) for parameter export without microdata.
• Reuses the helper from Simulation.get_baseline_params().
• Documents the dataset-free parameter path and adds hermetic wrapper tests.

Validation:

• PYTHONPATH=interfaces/python python -m pytest -q interfaces/python/tests/test_baseline_params.py
• PYTHONPATH=interfaces/python python -m pytest -q interfaces/python/tests/test_baseline_params.py interfaces/python/tests/test_parity_harness.py