Complete Köppen-Geiger filter review with Mixed climate class

Classify each county by area-weighted Köppen class shares: a county is
predominantly its top class when that class covers at least 50% of its
land and leads the runner-up by at least 5 percentage points; otherwise
it is Mixed (133 of 3,143 counties in the 50 states and DC).

- Add build_county_koppen_metric.py (writes data/metrics/koppen.csv) and
  apply_koppen_metric_to_climate_data.py (writes koppenZone plus
  koppenPrimaryClass/koppenSecondaryClass for Mixed counties).
- Move shared helpers into scripts/common/ (county loading, Köppen
  legend, area-weighted raster shares); fix the 180th-meridian raster
  window for Aleutians West.
- Add check_climate_data.py to validate the app CSV.
- Draw Mixed counties in app.js as diagonal stripes of their top two
  classes, fixed to the ground and following the map at every zoom, with
  a crossfade only when the stripe size changes. Filtering a class also
  matches Mixed counties where it is primary or secondary.
- Document the rule, display, and pipeline plan in docs/ and update the
  README and data-source notes.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/test_koppen_antimeridian.py

@@ -0,0 +1,104 @@
+from __future__ import annotations
+
+import sys
+import unittest
+from pathlib import Path
+from tempfile import TemporaryDirectory
+
+import geopandas as gpd
+import numpy as np
+import rasterio
+from rasterio.transform import from_origin
+from shapely.geometry import MultiPolygon, Polygon, box
+
+SCRIPTS_DIR = Path(__file__).resolve().parents[1] / "scripts"
+sys.path.insert(0, str(SCRIPTS_DIR))
+
+from build_county_climate_data import _touched_raster_values, _zonal_majority_class  # noqa: E402
+from common.county_zonal_stats import geometry_window, split_at_antimeridian  # noqa: E402
+from common.koppen_legend import DEFAULT_KOPPEN_CODE_MAP  # noqa: E402
+
+
+def islands_on_both_sides() -> MultiPolygon:
+    """A county shaped like Aleutians West: islands at +178..+180 and -180..-179."""
+    return MultiPolygon([box(178.2, 51.2, 179.8, 51.8), box(-179.8, 51.2, -179.2, 51.8)])
+
+
+class SplitAtAntimeridianTests(unittest.TestCase):
+    def test_geometry_on_one_side_is_unchanged(self) -> None:
+        geometry = box(-100.0, 30.0, -90.0, 40.0)
+
+        pieces = split_at_antimeridian(geometry)
+
+        self.assertEqual(len(pieces), 1)
+        self.assertTrue(pieces[0].equals(geometry))
+
+    def test_islands_are_grouped_by_side(self) -> None:
+        geometry = MultiPolygon(
+            [box(172.0, 51.0, 173.0, 52.0), box(179.0, 51.0, 179.5, 52.0), box(-179.0, 51.0, -178.0, 52.0)]
+        )
+
+        pieces = split_at_antimeridian(geometry)
+
+        self.assertEqual(len(pieces), 2)
+        bounds = sorted(piece.bounds for piece in pieces)
+        self.assertEqual(bounds[0], (-179.0, 51.0, -178.0, 52.0))
+        self.assertEqual(bounds[1], (172.0, 51.0, 179.5, 52.0))
+        self.assertAlmostEqual(sum(piece.area for piece in pieces), geometry.area)
+
+    def test_single_outline_crossing_the_line_is_cut(self) -> None:
+        # Encoded in -180..180 coordinates, this 2-degree-wide ring looks 358 degrees wide.
+        crossing = Polygon([(179.0, 50.0), (-179.0, 50.0), (-179.0, 51.0), (179.0, 51.0)])
+
+        pieces = split_at_antimeridian(crossing)
+
+        bounds = sorted(piece.bounds for piece in pieces)
+        self.assertEqual(bounds, [(-180.0, 50.0, -179.0, 51.0), (179.0, 50.0, 180.0, 51.0)])
+        self.assertAlmostEqual(sum(piece.area for piece in pieces), 2.0)
+
+
+class WindowedKoppenTests(unittest.TestCase):
+    def setUp(self) -> None:
+        self._temp_dir = TemporaryDirectory()
+        self.raster_path = Path(self._temp_dir.name) / "koppen.tif"
+        # Global 1-degree raster; row 38 covers 51..52 N.
+        data = np.zeros((180, 360), dtype=np.uint8)
+        data[38, 358] = 29  # ET at 178..179 E
+        data[38, 359] = 29  # ET at 179..180 E
+        data[38, 0] = 15  # Cfb at 180..179 W
+        with rasterio.open(
+            self.raster_path,
+            "w",
+            driver="GTiff",
+            height=180,
+            width=360,
+            count=1,
+            dtype="uint8",
+            crs="EPSG:4326",
+            transform=from_origin(-180.0, 90.0, 1.0, 1.0),
+            nodata=0,
+        ) as destination:
+            destination.write(data, 1)
+
+    def tearDown(self) -> None:
+        self._temp_dir.cleanup()
+
+    def test_each_side_is_read_from_a_small_window(self) -> None:
+        with rasterio.open(self.raster_path) as source:
+            windows = [geometry_window(source, piece) for piece in split_at_antimeridian(islands_on_both_sides())]
+            values = _touched_raster_values(source, islands_on_both_sides(), split_antimeridian=True)
+
+        self.assertEqual(len(windows), 2)
+        self.assertTrue(all(window.width <= 5 for window in windows))
+        self.assertEqual(sorted(values[values != 0].tolist()), [15, 29, 29])
+
+    def test_majority_class_counts_islands_on_both_sides(self) -> None:
+        counties = gpd.GeoDataFrame(geometry=[islands_on_both_sides(), box(-100.0, 30.0, -99.0, 31.0)], crs="EPSG:4326")
+
+        classes = _zonal_majority_class(self.raster_path, counties, DEFAULT_KOPPEN_CODE_MAP)
+
+        self.assertEqual(classes, ["ET", "Cfa"])
+
+
+if __name__ == "__main__":
+    unittest.main()