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/README.md

@@ -0,0 +1,39 @@
+# `scripts/common/`
+
+`scripts/common/` holds code used by more than one data source (NOAA, gridMET,
+NSRDB, Köppen). Scripts import from it, for example
+`from common.counties import load_counties`; nothing in it is run directly.
+
+Rules for `common/`:
+
+- **Cross-source only.** A helper goes in only if metrics from more than one
+  data source use it. Code shared by scripts of a single data source stays with
+  that source, for example a future NSRDB module for the NSRDB prompt,
+  redaction, and error-log helpers.
+- **One topic per module.** Each module is named for its topic and has a
+  docstring. No catch-all `utils.py`.
+- **Keep it small.** Before adding a helper, ask why it does not belong to any
+  one data source.
+
+Other shared code moves when its filter is reviewed, so each move is tested
+alongside that filter. Duplicated and drifted helpers found by the 2026-09-13
+survey are finding 14 in
+[docs/reviews/00-cross-filter.md](../../docs/reviews/00-cross-filter.md).
+
+Current modules:
+
+| Module | Contents | Why it is in `common/` |
+| --- | --- | --- |
+| `county_zonal_stats.py` | Raster windows, the 180th-meridian split, area-weighted class shares | Used by any raster-based metric |
+| `counties.py` | County polygon loading, FIPS normalization, the state FIPS table | County identity is shared by nearly every pipeline |
+| `koppen_legend.py` | The Köppen code map and legend loader | Temporary: also used by `build_county_climate_data.py`; moves next to the Köppen code in Phase 3 |
+
+**Rationale.** Helper functions make each step of a computation explicit,
+avoid repeated code, and can be tested separately
+([Brown CSCI 0111, "Helper Functions"](https://cs.brown.edu/courses/csci0111/fall2018/lectures/helper-functions.html)).
+Shared helper folders, however, tend to lose cohesion and collect unrelated
+code; the recommended alternative is to keep code with the part of the system
+it belongs to, allowing a shared folder only if it stays small and documented
+([Helpers and Utils Folders in Software Architecture](https://dev.to/knzt/helpers-and-utils-folders-in-software-architecture-3f8h)).
+The rules above follow both: shared functions, organized by topic and limited
+to code that crosses data sources.