feat(dashboard): CMU's 2024 flood marks as a map layer

CMU Water Center marked the October 2024 high-water line at ~4,900 places in
Chiang Mai and Lamphun (depth above the road while P.1 stood at 5.30 m).

- src/static/flood-marks-2024.json (position and depth, 110 KB), built by
  scripts/build_flood_marks.py from CMU's GeoJSON.
- "Flood marks 2024" layer: canvas dots from zoom 12 in CMU's five depth
  classes, legend row, CMU credit; turning it on below zoom 12 flies to the
  city. A mark's popup gives the depth and its flood zone.
- Zone and marker popups name the nearest mark within 250 m (loaded with the
  zones).
- No depth estimate for other P.1 levels: the marks do not scale one-to-one
  with P.1 (marks in the 5.30 m zone stood a median 65 cm deep in 2024).
- The layer bar wraps on narrow maps; popups pan clear of it.
- Docs: DATA_SOURCES 5.1, DASHBOARD_REDESIGN, README.

docs/DATA_SOURCES.md

@@ -250,7 +250,12 @@ and the province). A Next.js app; its data paths are in the page chunks
   (`uv run scripts/build_flood_zones.py`).
 - **Flood marks 2024** — `/data/New_floodMark_2025_fixed_like_pole.geojson` (4.9 MB):
   4,869 points in Chiang Mai and Lamphun with the October 2024 depth above the road
-  in cm (P.1 = 5.30 m), the marker's place and type.
+  in cm (P.1 = 5.30 m), the marker's place and type. **In use:** the dashboard's
+  "Flood marks 2024" layer, `src/static/flood-marks-2024.json` (position and depth
+  only, 110 KB; `uv run python scripts/build_flood_marks.py`). The depth does not
+  scale one-to-one with P.1: marks in the zone that floods only at 5.30 m stood a
+  median 65 cm deep in 2024, so the site shows the 2024 depth and no estimate for
+  other levels.
 - **Flood poles** — `/data/PoleCNX2025_v2.geojson`: 219 poles with ground level and
   maximum flood level (m MSL) and the depth.
 - **City-reach gauges** (station page): P.67, P.103, P.1, P.104 (RID, `ST.16`
@@ -279,7 +284,8 @@ and the province). A Next.js app; its data paths are in the page chunks
 | Open-Meteo rain | ✅ Hourly by ping-collector → `openmeteo_rain` (5 catchment points, past 5 days + 48 h forecast); the 2021+ archive trains the hgb-v3 rain features | — |
 | HII API key (dams, forecasts) | ❌ | Contact HII for sanctioned access |
 | CMU CMFlood flood map (2024) | ✅ Static (2026-10-25): P.1 stages and the city's flood zones | Rebuild with `scripts/build_flood_zones.py` if CMU revises the map |
-| CMU CMFlood marks, poles, city gauges | ❌ | See 5.1 |
+| CMU CMFlood flood marks 2024 | ✅ Static (2026-10-25): dashboard layer, nearest mark in zone and marker popups | Rebuild with `scripts/build_flood_marks.py` |
+| CMU CMFlood poles, city gauges | ❌ | See 5.1 |
 
 **Collector configuration** (ping-collector): HII, the dams and Open-Meteo are
 fetched once per clock hour, also while RID is being retried; one-shot via