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/01-koppen.md

@@ -6,7 +6,7 @@ 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).
+[koppen-mixed-display.md](../koppen-mixed-display.md).
 
 ## Findings
 
@@ -69,7 +69,7 @@ covers at least 50% of the county's land and leads the runner-up by at least
 - [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).
+      [koppen-mixed-display.md](../koppen-mixed-display.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