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>

docs/koppen-mixed-display-plan.md

@@ -9,7 +9,8 @@ and the project documentation was updated (section 6).
 **Scope:** Show Köppen-Geiger "Mixed" counties as striped on the map, filter them
 by their top two classes, and carry the two stripe classes from the pipeline into
 the app. The classification rule itself is described in
-[filter-calculations.md](filter-calculations.md) §1; overall progress is tracked in
+[filter-calculations.md](filter-calculations.md) §1; the filter's review is
+[reviews/01-koppen.md](reviews/01-koppen.md), and overall progress is tracked in
 [pipeline-plan.md](pipeline-plan.md).
 
 ## 1. Design