Refine project documentation and track metric files

Close gaps found in a review of the documentation:
- Track data/metrics/ and data/metric_sources.json in git so the data
  checker passes on a fresh clone (finding 23; decision logged)
- State the Alaska and Hawaii coverage gap in the README limitations and
  extend finding 12
- File findings 24-26: the data-sources doc lacks gridMET and several
  pipeline commands; wettest/driest month are computed twice; solar GHI
  is written by the extreme-temperature apply step
- Add the stale "fallback values" note to filter 2's tasks

Tidy the document system:
- Add docs/reviews/README.md with the numbering rules and a finding index
- Rename koppen-mixed-display-plan.md to koppen-mixed-display.md and fix
  its stale Puerto Rico and "stage 5" text
- Add the precipitation-month step to the README enrichment list
- Describe the Current method / Previous method pattern in plan section 5
- Add CLAUDE.md with the project guardrails and doc layout

Format filter-calculations.md so it renders on GitHub and in VS Code:
inline math uses $...$, ranges use en dashes, and implementation
references name functions instead of line numbers.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/reviews/00-cross-filter.md

@@ -1,10 +1,7 @@
 # Cross-filter review
 
-Findings that affect more than one filter. Each finding lives in exactly one
-review file; the filter reviews it affects link here and record only how it
-applied to them. Findings keep their numbers across all review files, and new
-findings take the next number wherever they are filed. A finding that turns out
-to affect other filters moves here, leaving a link behind.
+Findings that affect more than one filter. The numbering conventions and an
+index of every finding are in [README.md](README.md).
 
 ## Findings
 
@@ -19,7 +16,7 @@ Affects filters 1 (resolved), 2, 6, and 7.
 **11. Spatial weighting is inconsistent across metric families.** Base NOAA
 normals use equal touched-cell weights, Köppen uses area-weighted class
 shares, gridMET humidity uses
-\(\cos(\phi)\) weights on cell centers, and NSRDB polygon metrics use
+$\cos(\phi)$ weights on cell centers, and NSRDB polygon metrics use
 estimated overlap areas. Cross-metric comparisons should account for these
 different county aggregation methods.
 
@@ -35,7 +32,12 @@ clear-sky reduction (filters 11 and 12) cover both states.
 `check_climate_data.py` allows these blanks (`OUTSIDE_CONUS`). The notes on
 the base build in `scripts/county_data_sources.md` say "fallback values are
 applied" for counties outside NOAA coverage, but `build_county_climate_data.py`
-leaves them blank (lines 499–520).
+leaves them blank (`build_county_records`).
+
+The README's "Current Limitations" states the gap (added 2026-09-15), but the
+app's county panel shows only "No data". If these counties stay blank, the app
+should give the reason, for example "Not covered: source data is contiguous
+U.S. only".
 
 Affects filters 2–10. Open decision: "Alaska and Hawaii coverage" in
 [decisions.md](../decisions.md).
@@ -99,6 +101,56 @@ neither column is in `data/climate-data.csv`. The list has no entry for
 
 Affects filters 3 and 4.
 
+**23. The metric files and `metric_sources.json` were not in git.**
+`.gitignore` excluded everything under `data/` except the app CSV and the
+county GeoJSON, so `data/metrics/koppen.csv` and `data/metric_sources.json`
+existed only locally. On a fresh clone, `check_climate_data.py` failed its
+metric sources check, and reproduction tier 2 in
+[pipeline-plan.md](../pipeline-plan.md) (reassemble from committed metric
+files) was impossible. Found 2026-09-15. *Resolved on 2026-09-15: `.gitignore`
+now keeps both; see "Committing metric files" in
+[decisions.md](../decisions.md).*
+
+Affects every filter, since each adds a metric file.
+
+**24. The data-sources doc does not cover several pipeline steps.**
+`scripts/county_data_sources.md` has source sections for Köppen, the NOAA
+gridded normals, county geometry, NSRDB solar, and nClimGrid-Daily, but none
+for gridMET: no dataset link, license, variables, or commands for
+`download_gridmet_data.py` → `summarize_county_gridmet_humidity.py` →
+`apply_gridmet_humidity_metric_to_climate_data.py`. gridMET appears only in two
+metric-definition lines. The doc also has no commands for
+`build_county_diurnal_temperature_range.py` /
+`apply_diurnal_temperature_range_to_climate_data.py` or
+`apply_precipitation_month_metrics_to_climate_data.py`. Found 2026-09-15.
+
+Affects filters 3, 5, 8, 9, and 10; each filter's review adds its section or
+commands.
+
+**25. Wettest and driest month are computed in two places.**
+`build_county_climate_data.py` writes `wettestPrecipMonth` and
+`driestPrecipMonth` (`_precip_month_extremes`), and
+`apply_precipitation_month_metrics_to_climate_data.py` recomputes and
+overwrites them (`build_precip_month_lookup`), reusing the base build's
+climatology and zonal-mean helpers. The live values are correct only if the
+second script runs after the base build. Noted in the original review;
+numbered 2026-09-15.
+
+Affects filters 8 and 9. This is part of problem 3 in
+[pipeline-plan.md](../pipeline-plan.md) §1.
+
+**26. Solar GHI is finalized by the extreme-temperature apply step.**
+`build_county_climate_data.py` can write
+`meanDailyGlobalHorizontalRadiationKwhM2Day`, but the live value is written by
+`apply_locally_extreme_metric_to_climate_data.py`, which prefers polygon GHI
+and falls back to representative-point GHI. Updating GHI therefore means
+rerunning the extreme-temperature apply step, and nothing in that script's name
+says it owns the solar column. Noted in the original review; numbered
+2026-09-15.
+
+Affects filters 4 and 11. This is part of problem 3 in
+[pipeline-plan.md](../pipeline-plan.md) §1.
+
 ## Decisions
 
 In [decisions.md](../decisions.md): Shared helpers (2026-09-13), and the open
@@ -115,7 +167,8 @@ scripts, not solely from UI descriptions. No calculation code was changed. The
 automated test suite was not executed during this review because `pytest` is
 not installed in either the system Python environment or the project virtual
 environment. The suite uses `unittest` and runs without pytest:
-`python -m unittest discover -s tests`.
+`.venv\Scripts\python.exe -m unittest discover -s tests`. As of 2026-09-15,
+all 74 tests pass.
 
 Section 1 and findings 2, 4, and 11 were updated on 2026-09-14, after the
 Köppen classification was reworked and applied.