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/common/koppen_legend.py

@@ -1,4 +1,4 @@
-"""Koppen-Geiger raster codes and the Beck et al. legend loader."""
+"""Köppen-Geiger raster codes and the Beck et al. legend loader."""
 
 from __future__ import annotations
 
@@ -42,7 +42,7 @@ DEFAULT_KOPPEN_CODE_MAP = {
 
 
 def load_koppen_legend(legend_path: Path | None) -> Dict[int, str]:
-    """Load Koppen raster codes, using defaults when no legend exists."""
+    """Load Köppen raster codes, using defaults when no legend exists."""
     if legend_path is None:
         return DEFAULT_KOPPEN_CODE_MAP