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/decisions.md

@@ -0,0 +1,45 @@
+# Decisions
+
+Open decisions and a dated log of decided ones for the pipeline and the
+filter-by-filter review. The findings that led to them are in
+[reviews/](reviews/); the plan they serve is [pipeline-plan.md](pipeline-plan.md).
+
+## Open decisions
+
+| Decision | Options | Needed by |
+| --- | --- | --- |
+| Committing large intermediates | Commit metric files only, or also source summaries | Phase 3 |
+| Alaska and Hawaii coverage | Leave blank with a stated coverage gap, or add a source that covers them (for example Daymet or TerraClimate). NOAA nClimGrid and gridMET cover the contiguous U.S. only, so 29 AK and 5 HI counties are blank in filters 2–10; see finding 12 in [reviews/00-cross-filter.md](reviews/00-cross-filter.md) | Before Phase 3 |
+
+## Decided
+
+- **Köppen audit columns (2026-09-12):** `koppen.csv` stores the top class and
+  share and the runner-up class and share alongside `koppenZone`.
+- **Köppen no-data fallback (2026-09-12):** a county with no valid raster cells
+  is left blank, not assigned `Cfa`.
+- **Applying Köppen to the app CSV (2026-09-12):** wait until the app supports
+  the Mixed class. Done 2026-09-13.
+- **Metric files (2026-09-13):** one CSV per metric under `data/metrics/`,
+  starting with `koppen.csv`.
+- **Mixed climate display (2026-09-13):** diagonal stripes of each Mixed
+  county's top two classes; see
+  [koppen-mixed-display-plan.md](koppen-mixed-display-plan.md).
+- **Puerto Rico (2026-09-13):** off the map and out of every filter. The app
+  already drops state FIPS 72; the data files keep the rows.
+- **Shared helpers (2026-09-13):** `scripts/common/` holds only code used by
+  more than one data source, one topic per module; code shared within one data
+  source stays with that source. Duplicates move during their own filter's
+  review.
+- **Annual avg temperature month weighting (2026-09-15):** the annual value is
+  the equally weighted mean of the 12 monthly normals, not weighted by month
+  length. This follows WMO-No. 1203 (2017) §4.3.3(a), whose footnote
+  recommends against day-length weighting, and NOAA's 1991–2020 Normals
+  methodology (p. 2). Day weighting would have shifted values by only
+  +0.03 to +0.13 °F. Closes finding 1.
+- **Annual avg temperature aggregation (2026-09-15):** area-weighted county
+  means (sub-cell coverage fraction × cos(latitude)), matching Köppen. Closes
+  finding 2 for this metric.
+- **Annual avg temperature completeness (2026-09-15):** a county is blank
+  unless all 12 monthly values are present (WMO-No. 1203 §4.3.3).
+- **Annual avg temperature in Alaska and Hawaii (2026-09-15):** left blank for
+  now; tracked as the cross-filter "Alaska and Hawaii coverage" open decision.