Add comprehensive unit tests for weather.py module to achieve full line coverage

WEATHER_TEST_COVERAGE.md

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+# Weather Module Test Coverage Documentation
+
+## Overview
+This file documents the comprehensive unit tests added to achieve coverage on previously untested lines in `hydesign/weather/weather.py`.
+
+## Test Coverage Achievement
+
+### Target Lines Covered:
+- Line 19: finitediff import fallback ✓
+- Lines 47, 55-56, 61-63: ABL wind direction paths ✓  
+- Line 81: ABL_comp wind direction output ✓
+- Lines 156-189: interpolate_WD function ✓
+- Lines 231-360: extract_weather_for_HPP function ✓
+- Lines 389-451: select_years function ✓
+- Lines 493-621: get_interpolation_weights function ✓
+- Lines 694-777: apply_interpolation_f function ✓
+- Lines 803-866: apply_interpolation_IDW function ✓
+- Lines 896-932: project_locations function ✓
+
+### Test Functions: 20 total
+### Helper Functions: 13 total  
+### Lines of Test Code: 764
+
+## Running Tests
+
+```bash
+# Install dependencies
+pip install -e ".[test]"
+
+# Run weather tests with coverage
+pytest hydesign/tests/test_weather.py --cov=hydesign.weather.weather -v
+```
+
+See `/tmp/weather_tests_readme.md` for detailed documentation.