feat(p1): city flood stages and zones from CMU Water Center's 2024 flood map

CMU Water Center's Chiang Mai flood hazard map (CMFlood) replaces the older
municipal map. It is built on the October 2024 flood and has the Ping leave
its banks above P.1 4.20 m, with five stages: 4.30, 4.50, 4.70, 5.00 and
5.30 m (the 2024 record). The old map's seven stages (3.70-4.60 m) said the
city floods from 3.70 m; CMU calls that map out of date.

- P1_FLOOD_STAGES: the five CMU stages, discharge on RID's 2024 rating.
  /forecast's P.1 `stages` now has five entries (same format). Thresholds
  are unchanged: 4.20 m danger is the bank top, 3.70 m warning half a metre
  below (as on CMU's gauge page). flood_config.json and the Rust parity
  fixtures regenerated (only the stages changed).
- flood-zones-p1.geojson: CMU's KML (L1..L5) as five disjoint zones, zone k
  floods first at stage k; scripts/build_flood_zones.py rebuilds it.
- Dashboard: CMU's colours and credit (map attribution); the stage grid follows
  the number of stages; zone lookup for markers honours holes. The city
  verdict uses statusOf's classes: flooding (over the banks) from 4.20 m,
  high water from 3.70 m. Zones show by themselves when the river is over its
  banks or stage 1 is >= 50 % likely. Alert, replay and verdict texts no
  longer call 3.70 m city flooding (EN + TH).
- Docs: DATA_SOURCES 5.1 (CMFlood: the map, flood marks, poles, city gauges),
  FLOOD_FORECASTING, DASHBOARD_REDESIGN, NOTIFICATIONS, README credits.

docs/FLOOD_FORECASTING.md

@@ -465,15 +465,19 @@ validated.
 **Thresholds are per-station as of 2026-08-10.** `THRESHOLDS` now carries
 calibrated (warning, danger) pairs for all 16 stations, derived from the DB's
 `discharge_percent` (RID % of channel capacity): warning = median level at
-75–85% capacity, danger = median level at 95–105%. P.1 instead uses the official
-Chiang Mai inundation map (`P1_FLOOD_STAGES`): warning 3.70 m (city flooding
-begins, stage 1) and danger 4.20 m (stage 5). The prior single default of
+75–85% capacity, danger = median level at 95–105%. P.1 is keyed to its banks:
+danger 4.20 m, where the Ping overflows them (CMU Water Center's flood map of the
+2024 flood; the lower bank is 304.65 m MSL = 4.15 m on the gauge), and warning
+3.70 m, half a metre below. (Until 2026-10-25 the docs called 3.70 m "city flooding
+begins", after the older municipal map; the values did not change.) The prior single default of
 (3.0, 4.5) m made P.103 badly over-alert (its bank-full level is ~6.75 m) and
 P.67 under-alert (overflow at ~2.9 m, 1.6 m below the old danger line).
 **A retrain is required after any threshold change** — classifier labels depend
 on them; until then, model rows report the thresholds baked into their bundle.
-P.1 additionally reports `stages`: exceedance probability for each of the seven
-official inundation stages (3.70–4.60 m), computed from the regression head and
+P.1 additionally reports `stages`: exceedance probability for each of the five
+city flood stages (`P1_FLOOD_STAGES`, 4.30–5.30 m, from CMU Water Center's flood
+hazard map of the October 2024 flood; the seven stages of the older municipal map,
+3.70–4.60 m, were replaced on 2026-10-25), computed from the regression head and
 its calibration sigma, so they need no retrain and no per-stage classifiers.
 
 ### 2026-08-13: Mae Ngat dam features — a documented negative result
@@ -661,14 +665,16 @@ and `scripts/install.sh` does that on every install.
 The dashboard (`src/static/dashboard.html`) loads `/forecast` with the gauges; if
 it fails or is empty, the page works without forecasts. Where the model shows:
 
-- **Top bar.** Chiang Mai's verdict from P.1: flooding at ≥ 3.70 m; "likely" at a
-  24 h danger chance ≥ 50 % or warning chance ≥ 70 %; "watch" at a warning chance
-  ≥ 20 % or a level ≥ 3.20 m. Next to it, the highest 24 h warning chance of any gauge.
+- **Top bar.** Chiang Mai's verdict from P.1, in the classes of the gauge status:
+  flooding (over the banks) at ≥ 4.20 m; high water at ≥ 3.70 m; "flooding likely" at
+  a 24 h danger chance ≥ 50 %, "high water likely" at a warning chance ≥ 70 %;
+  "watch" at a warning chance ≥ 20 % or a level ≥ 3.20 m. Next to it, the highest
+  24 h warning chance of any gauge.
 - **Gauge list and map.** A gauge is "watch" from a 24 h warning chance of 25 %.
 - **Station panel.** Per horizon (6/12/24 h) the predicted peak and the warning and
   danger chances, the model version and when it was trained, or "Simple estimate" when
   the row came from the heuristic fallback. For P.1 the chance of reaching each of the
-  seven inundation stages. The history chart overlays the 24 h peaks as they were
+  five city flood stages. The history chart overlays the 24 h peaks as they were
   issued (`/api/forecast/history`), and "How accurate is the forecast?" loads
   `/api/forecast/skill`.
 - **Replay.** "Replay the October 2024 flood" plays `src/static/flood-2024-all.json`: