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>

scripts/check_climate_data.py

@@ -110,7 +110,7 @@ METRIC_RULES: Dict[str, MetricRule] = {
 }
 
 IDENTITY_COLUMNS = ("countyFips", "countyName", "state")
-# Stripe classes the app draws for Mixed Koppen counties; blank otherwise.
+# Stripe classes the app draws for Mixed Köppen counties; blank otherwise.
 KOPPEN_STRIPE_COLUMNS = ("koppenPrimaryClass", "koppenSecondaryClass")
 AUDIT_COLUMNS = ("humidHeatSourceFips", "humidHeatFipsAdjustment", "source")
 EXPECTED_COLUMNS = IDENTITY_COLUMNS + tuple(METRIC_RULES) + KOPPEN_STRIPE_COLUMNS + AUDIT_COLUMNS
@@ -341,13 +341,13 @@ def check_cross_fields(report: Report, rows: List[Dict[str, str]]) -> None:
         secondary = row.get("koppenSecondaryClass") or ""
         if zone == MIXED_KOPPEN_CLASS:
             if not (primary and secondary):
-                report.add(CHECK_CROSS, f"{fips}: Mixed Koppen county needs koppenPrimaryClass and koppenSecondaryClass")
+                report.add(CHECK_CROSS, f"{fips}: Mixed Köppen county needs koppenPrimaryClass and koppenSecondaryClass")
             elif primary not in KOPPEN_CODES or secondary not in KOPPEN_CODES:
-                report.add(CHECK_CROSS, f"{fips}: invalid Koppen stripe classes {primary!r}/{secondary!r}")
+                report.add(CHECK_CROSS, f"{fips}: invalid Köppen stripe classes {primary!r}/{secondary!r}")
             elif primary == secondary:
-                report.add(CHECK_CROSS, f"{fips}: Koppen stripe classes are both {primary}")
+                report.add(CHECK_CROSS, f"{fips}: Köppen stripe classes are both {primary}")
         elif primary or secondary:
-            report.add(CHECK_CROSS, f"{fips}: Koppen stripe classes are set but koppenZone is {zone or 'blank'}, not Mixed")
+            report.add(CHECK_CROSS, f"{fips}: Köppen stripe classes are set but koppenZone is {zone or 'blank'}, not Mixed")
 
         if not (row.get("source") or "").strip():
             report.add(CHECK_CROSS, f"{fips}: source is blank")