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>

scripts/common/koppen_legend.py

@@ -0,0 +1,64 @@
+"""Koppen-Geiger raster codes and the Beck et al. legend loader."""
+
+from __future__ import annotations
+
+import re
+from pathlib import Path
+from typing import Dict
+
+# Beck et al legend key is expected as text file, but this default handles common codes.
+DEFAULT_KOPPEN_CODE_MAP = {
+    1: "Af",
+    2: "Am",
+    3: "Aw",
+    4: "BWh",
+    5: "BWk",
+    6: "BSh",
+    7: "BSk",
+    8: "Csa",
+    9: "Csb",
+    10: "Csc",
+    11: "Cwa",
+    12: "Cwb",
+    13: "Cwc",
+    14: "Cfa",
+    15: "Cfb",
+    16: "Cfc",
+    17: "Dsa",
+    18: "Dsb",
+    19: "Dsc",
+    20: "Dsd",
+    21: "Dwa",
+    22: "Dwb",
+    23: "Dwc",
+    24: "Dwd",
+    25: "Dfa",
+    26: "Dfb",
+    27: "Dfc",
+    28: "Dfd",
+    29: "ET",
+    30: "EF",
+}
+
+
+def load_koppen_legend(legend_path: Path | None) -> Dict[int, str]:
+    """Load Koppen raster codes, using defaults when no legend exists."""
+    if legend_path is None:
+        return DEFAULT_KOPPEN_CODE_MAP
+
+    mapping: Dict[int, str] = {}
+    for line in legend_path.read_text(encoding="utf-8").splitlines():
+        text = line.strip()
+        if not text or text.startswith("#"):
+            continue
+        # Handles patterns like:
+        # "1:  Af ..." or "1 = Af" or "1 Af"
+        match = re.match(r"^(\d+)\s*[:=]?\s*([A-Za-z]{2,3})\b", text)
+        if not match:
+            continue
+
+        key = int(match.group(1))
+        value = match.group(2)
+        mapping[key] = value
+
+    return mapping if mapping else DEFAULT_KOPPEN_CODE_MAP