Document restructuring and the beginnings of Filter 2 changes

Split the pipeline documentation by purpose so each fact has one home:
- docs/pipeline-plan.md keeps the plan, checklist, tracker, and guardrails
- docs/decisions.md holds open decisions and the dated decision log
- docs/reviews/ holds findings and tasks: one file per filter, plus
  00-cross-filter.md for findings that span filters
- scripts/common/README.md holds the shared-helper rules (formerly Phase 2)
- filter-calculations.md now describes calculations only

Filed findings 12-22 from a consistency audit of the app, docs, and scripts.

Filter 1 (Köppen-Geiger): use "Köppen" with the umlaut in all prose, labels,
docstrings, help text, and checker messages (finding 21), and correct the
base build's "majority" docstring (finding 22).

Filter 2 (annual avg temperature): record the adopted definition in
filter-calculations.md §2: equally weighted 1991-2020 monthly normals, per
WMO-No. 1203 and NOAA's 2020 methodology; area-weighted county means; blank
unless all 12 months exist. Code changes for this filter are still pending.

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

app.js

@@ -106,7 +106,7 @@ const KOPPEN_CLASS_META = {
   Mixed: { label: "Mixed Climate", color: "#9aa3b2" }
 };
 
-// Counties with no predominant Koppen-Geiger class; see docs/koppen-mixed-display-plan.md.
+// Counties with no predominant Köppen-Geiger class; see docs/koppen-mixed-display-plan.md.
 const KOPPEN_MIXED_CLASS = "Mixed";
 // Standard style: share of each stripe pair given to the primary (top) class; the secondary class
 // gets the rest.
@@ -147,17 +147,17 @@ const MONTH_CATEGORY_META = {
 
 const METRICS = {
   koppenZone: {
-    label: "Koppen-Geiger Climate Class",
+    label: "Köppen-Geiger Climate Class",
     type: "categorical",
     unit: "class",
     categoryMeta: KOPPEN_CLASS_META,
-    allOptionLabel: "All Koppen Classes",
+    allOptionLabel: "All Köppen Classes",
     source: {
       description:
-        "Beck et al. 2023 Koppen-Geiger maps (1991-2020, 1 km). County class shares are area-weighted. A county shows 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 Climate, striped in the colors of its top two classes.",
+        "Beck et al. 2023 Köppen-Geiger maps (1991-2020, 1 km). County class shares are area-weighted. A county shows 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 Climate, striped in the colors of its top two classes.",
       links: [
         {
-          label: "Beck et al. 2023 Koppen-Geiger",
+          label: "Beck et al. 2023 Köppen-Geiger",
           url: "https://www.nature.com/articles/s41597-023-02549-6"
         }
       ]
@@ -389,7 +389,7 @@ const COUNTY_DETAIL_METRIC_KEYS = [...DATA_METRIC_KEYS];
 const METRIC_GROUPS = [
   {
     key: "koppenGeigerClassification",
-    label: "Koppen-Geiger Classification",
+    label: "Köppen-Geiger Classification",
     metricKeys: ["koppenZone"]
   },
   {
@@ -733,7 +733,7 @@ function normalizeAntimeridianFeature(feature) {
   });
 }
 
-// Clean and validate a Koppen-Geiger climate class code.
+// Clean and validate a Köppen-Geiger climate class code.
 function normalizeKoppenCode(code) {
   if (typeof code !== "string") {
     return null;
@@ -923,7 +923,7 @@ function sanitizeOverrideRecord(rawRecord) {
     }
   });
 
-  // Stripe classes for Mixed Koppen counties; blank for predominant counties.
+  // Stripe classes for Mixed Köppen counties; blank for predominant counties.
   sanitizedRecord.koppenPrimaryClass = normalizeKoppenCode(rawRecord.koppenPrimaryClass);
   sanitizedRecord.koppenSecondaryClass = normalizeKoppenCode(rawRecord.koppenSecondaryClass);
 
@@ -1166,7 +1166,7 @@ function getKoppenMixedStripePairWidthPx() {
   return pixelsPer100Miles / KOPPEN_MIXED_LINE_PAIRS_PER_100_MILES;
 }
 
-// Return a Mixed county's primary and secondary Koppen classes, or null when it is not striped.
+// Return a Mixed county's primary and secondary Köppen classes, or null when it is not striped.
 function getKoppenStripeClasses(stats) {
   if (stats?.koppenZone !== KOPPEN_MIXED_CLASS) {
     return null;
@@ -1421,7 +1421,7 @@ function createKoppenStripePatterns() {
   applyKoppenStripeStyle(getKoppenStripeStyle(map.getZoom()));
 }
 
-// Pick a county's map fill: a stripe pattern for Mixed Koppen counties, otherwise the value color.
+// Pick a county's map fill: a stripe pattern for Mixed Köppen counties, otherwise the value color.
 function fillForCounty(stats) {
   if (currentMetricKey === "koppenZone") {
     const classes = getKoppenStripeClasses(stats);
@@ -1442,7 +1442,7 @@ function buildKoppenMixedLegendSwatchBackground() {
   );
 }
 
-// Format a county's metric value, naming the stripe classes for Mixed Koppen counties.
+// Format a county's metric value, naming the stripe classes for Mixed Köppen counties.
 function formatCountyMetricValue(metricKey, stats) {
   const text = formatMetricValue(metricKey, stats[metricKey]);
   const classes = metricKey === "koppenZone" ? getKoppenStripeClasses(stats) : null;
@@ -1459,7 +1459,7 @@ function shouldFeaturePassFilter(stats) {
     if (currentCategoricalFilter === "all" || categoryValue === currentCategoricalFilter) {
       return true;
     }
-    // A Mixed Koppen county also matches its primary and secondary classes.
+    // A Mixed Köppen county also matches its primary and secondary classes.
     const stripeClasses = currentMetricKey === "koppenZone" ? getKoppenStripeClasses(stats) : null;
     return Boolean(stripeClasses) && [stripeClasses.primary, stripeClasses.secondary].includes(currentCategoricalFilter);
   }