eval: quantile heads and fc48 on top of hgb-v3 (rejected/deferred); HII gauge-rain aggregate

Rolling-origin harness gains rise_rain_quantile, rise_rain_quantile_uw,
rise_rain_qsigma (L2 point + quantile sigma) and rise_rain_fc48, all
opt-in, plus --from-cache for reproducible offline reruns. Results in
models/eval_2026-09-12*.json, write-up in docs/FLOOD_FORECASTING.md:

- quantile point prediction: better MAE, worse first-alert lead at 5 of
  11 events (P.103 2022-08-14 +6h -> +1h) -> rejected
- quantile sigma only: Brier within noise (0.0031 -> 0.0029) -> not worth 3x heads
- rain_fc48: neutral everywhere except 2024-10-03 P.1 (+21h -> +72h), n=1
  -> deferred to after the 2026 season

src/ml/hii_rain.py: catchment-mean hourly rain from the ~130 HII gauges in
the upper-Ping box and a 24h-sum comparison against Open-Meteo. Not a
training feature (table exists only since 2026-08-11, no archive); exposed
at GET /api/hii/rainfall/catchment so the two sources' agreement is on
record by the time a fold can test it.

data._read_cache now skips non-station files in models/cache/ (the shared
dir also holds rain_openmeteo / dam_* caches, which crashed the reader).
scripts/summarize_eval.py prints per-variant lead/peak-error tables.

tests/test_hii_collector.py

@@ -353,6 +353,29 @@ class TestHiiApiEndpoints:
         self._get(web_api, "get_hii_rainfall_latest", hours=48)
         assert calls["n"] == 2
 
+    def test_rainfall_catchment(self, web_api):
+        """One gauge in the box (CHM005, 19.12N 98.94E) is below the
+        MIN_GAUGES_PER_HOUR floor, so the catchment mean is NaN -> null, the
+        openmeteo_rain table does not exist in this store, and the comparison
+        reports no overlap. Shape is what matters: the endpoint must not 500
+        on a fresh database."""
+        payload, response = self._get(web_api, "get_hii_rainfall_catchment", days=7)
+        assert "x-data-stale" not in response.headers
+        assert list(payload) == ["box", "gauge", "openmeteo", "comparison_24h_sums"]
+        assert payload["openmeteo"] == []
+        assert payload["comparison_24h_sums"] == {"overlap_hours": 0}
+        assert len(payload["gauge"]) == 1
+        assert payload["gauge"][0]["rain_mm"] is None  # < MIN_GAUGES_PER_HOUR
+
+    def test_rainfall_catchment_disabled(self, monkeypatch):
+        from src import web_api
+
+        monkeypatch.setitem(web_api.app_state, "hii_collector", None)
+        web_api.HII_CACHE.clear()
+        web_api._REFRESH_IN_FLIGHT.clear()
+        payload, _ = self._get(web_api, "get_hii_rainfall_catchment", days=7)
+        assert payload["gauge"] == [] and payload["openmeteo"] == []
+
     def test_stale_served_on_recompute_failure(self, web_api, monkeypatch):
         # Prime the cache, expire it, break the DB: the stale copy is served
         # and flagged via the X-Data-Stale header.