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/reviews/01-koppen.md

@@ -0,0 +1,78 @@
+# Filter 1: Köppen-Geiger class
+
+**Status:** Done (2026-09-14): rule applied, Mixed display built, documentation
+updated.
+
+**Data keys:** `koppenZone`, `koppenPrimaryClass`, `koppenSecondaryClass`.
+Calculation: [filter-calculations.md](../filter-calculations.md) §1. Display
+design and history:
+[koppen-mixed-display-plan.md](../koppen-mixed-display-plan.md).
+
+## Findings
+
+**4. The Köppen fallback can create false data.** A county with no valid raster
+cells is labeled `Cfa` instead of missing. A null value plus an audit flag
+would distinguish missing coverage from a genuine humid-subtropical class.
+*Resolved on 2026-09-13: the Köppen builder leaves such counties blank. The
+fallback remains only in `build_county_climate_data.py`, whose Köppen
+value is replaced by the apply step.*
+
+**21. The documented Köppen labels differ from the app.** `filter-calculations.md`
+lists the group as "Climate Classification" and the filter as "Köppen-Geiger
+Climate Class"; `app.js` shows "Koppen-Geiger Classification" and
+"Koppen-Geiger Climate Class". The app spells Koppen without the umlaut
+throughout, while the docs use Köppen. *Resolved on 2026-09-15: every
+prose and display use of the name now reads "Köppen", in `app.js`,
+`README.md`, `scripts/county_data_sources.md`, script docstrings, help text,
+and checker messages (with the matching test expectations), and
+`filter-calculations.md` quotes the app's group label, "Köppen-Geiger
+Classification". Code identifiers, file names, and data paths such as
+`koppenZone`, `normalizeKoppenCode`, and `koppen_geiger_tif/` keep the ASCII
+spelling.*
+
+**22. The base build's docstring calls its Köppen value a majority class.**
+`build_county_climate_data.py` lists `koppenZone` as the "majority
+Koppen-Geiger class", but it assigns the most common class among touched
+cells, which need not be a majority. That value is replaced by the Köppen apply
+step, and the script is retired in Phase 3. *Resolved on 2026-09-15: the
+docstring now reads "largest-share Köppen-Geiger class among touched cells
+(replaced by the Köppen apply step)".*
+
+Cross-filter finding 2 (touched-cell aggregation) was resolved for Köppen on
+2026-09-13; see [00-cross-filter.md](00-cross-filter.md).
+
+## Decisions
+
+In [decisions.md](../decisions.md): Köppen audit columns, Köppen no-data
+fallback, and Applying Köppen to the app CSV (2026-09-12); Metric files and
+Mixed climate display (2026-09-13).
+
+## Tasks
+
+Adopted rule: a county is predominantly its top class if and only if that class
+covers at least 50% of the county's land and leads the runner-up by at least
+5 percentage points; otherwise it is Mixed climate. Expected result for the
+50 states and DC: 3,010 predominant, 133 Mixed.
+
+- [x] Investigate low-majority counties and adopt the rule.
+- [x] Windowed raster reads and 180th-meridian split.
+- [x] Area-weighted class shares (16 × 16 sub-cells per raster cell, scaled by
+      cos(latitude)) in `scripts/common/county_zonal_stats.py`.
+- [x] Apply the 50% / 5-point rule (`scripts/build_county_koppen_metric.py`;
+      counties with no valid cells are left blank).
+- [x] Run the builder to write `data/metrics/koppen.csv` and confirm the
+      expected 3,010 predominant / 133 Mixed (2026-09-13).
+- [x] `koppenZone`-only apply step
+      (`scripts/apply_koppen_metric_to_climate_data.py`, with `--dry-run`).
+- [x] Apply to `data/climate-data.csv` (2026-09-13; 142 counties changed to
+      Mixed, with `koppenPrimaryClass` and `koppenSecondaryClass` added).
+- [x] Allow `Mixed` in `check_climate_data.py`.
+- [x] Add a Mixed climate category to `app.js`, drawn as stripes of the
+      county's top two classes; see
+      [koppen-mixed-display-plan.md](../koppen-mixed-display-plan.md).
+- [x] Replace the plurality description in `filter-calculations.md` §1 and mark
+      review findings 2 and 4 resolved for Köppen (2026-09-14).
+- [x] Update the Köppen descriptions and script lists in `README.md` and
+      `scripts/county_data_sources.md` (2026-09-14).
+- [x] Tests for shares, the rule, boundary cases, and the apply step
+      (`tests/test_koppen_metric.py`).