feat(forecast): Rust forecast crate, bit-for-bit with the Python serving path

Phase 2a of docs/RUST_REWRITE.md. rust/forecast (ping-forecast) ports the
serving half of src/ml: the HII mirror gap fill, the hourly grid, every
feature (pandas' Kahan rolling sums included), the Mae Ngat dam frame,
HistGradientBoosting tree inference, the heuristic fallback, stale-threshold
handling and the P.1 flood-stage probabilities.

Hand-over from Python (src/ml/export.py):
- models/flood_<station>.json.gz next to each joblib bundle; train_all() writes
  it, retrain.sh promotes it, `python -m src.ml.export` exports existing ones.
- rust/forecast/flood_config.json: thresholds, stages, leads and notify
  constants, compiled in. tests/test_model_export.py fails while it differs
  from the Python source.

Parity (rust/forecast/tests/parity.rs, fixtures from
testdata/gen_forecast_fixtures.py on the production code path): four
scenarios incl. the 2024 flood with RID lagging behind HII. Merged readings,
all features and all levels identical; with all 15 local models 377/384
probabilities bit-identical, the rest within 2 ulp (exp).

Python refactor without behaviour change: predict.forecasts_from_long, and
get_latest_forecasts/load_latest/serving_frame take now/rain for
reproducible comparisons; the HII query orders its rows.

rust/forecast/flood_config.json

@@ -0,0 +1,348 @@
+{
+ "thresholds": {
+  "*": [
+   3.0,
+   4.5
+  ],
+  "P.1": [
+   3.7,
+   4.2
+  ],
+  "P.103": [
+   5.95,
+   6.75
+  ],
+  "P.20": [
+   2.35,
+   2.8
+  ],
+  "P.21": [
+   3.2,
+   3.6
+  ],
+  "P.4A": [
+   3.4,
+   3.9
+  ],
+  "P.5": [
+   4.55,
+   4.95
+  ],
+  "P.67": [
+   2.45,
+   2.9
+  ],
+  "P.75": [
+   3.2,
+   3.65
+  ],
+  "P.76": [
+   5.35,
+   5.45
+  ],
+  "P.77": [
+   4.3,
+   4.9
+  ],
+  "P.81": [
+   5.15,
+   6.3
+  ],
+  "P.82": [
+   3.4,
+   3.75
+  ],
+  "P.84": [
+   3.45,
+   3.9
+  ],
+  "P.85": [
+   2.9,
+   3.35
+  ],
+  "P.87": [
+   3.75,
+   4.05
+  ],
+  "P.92": [
+   2.95,
+   3.6
+  ]
+ },
+ "flood_stages": {
+  "P.1": [
+   {
+    "stage": 1,
+    "level": 3.7,
+    "discharge_cms": 405.0
+   },
+   {
+    "stage": 2,
+    "level": 3.9,
+    "discharge_cms": 438.0
+   },
+   {
+    "stage": 3,
+    "level": 4.0,
+    "discharge_cms": 458.0
+   },
+   {
+    "stage": 4,
+    "level": 4.1,
+    "discharge_cms": 478.0
+   },
+   {
+    "stage": 5,
+    "level": 4.2,
+    "discharge_cms": 493.0
+   },
+   {
+    "stage": 6,
+    "level": 4.3,
+    "discharge_cms": 508.0
+   },
+   {
+    "stage": 7,
+    "level": 4.6,
+    "discharge_cms": 558.0
+   }
+  ]
+ },
+ "upstream_leads": {
+  "P.1": [
+   [
+    "P.103",
+    1
+   ],
+   [
+    "P.67",
+    7
+   ],
+   [
+    "P.21",
+    9
+   ],
+   [
+    "P.75",
+    12
+   ],
+   [
+    "P.4A",
+    12
+   ],
+   [
+    "P.92",
+    15
+   ],
+   [
+    "P.20",
+    17
+   ]
+  ],
+  "P.103": [
+   [
+    "P.67",
+    6
+   ],
+   [
+    "P.21",
+    8
+   ],
+   [
+    "P.75",
+    11
+   ],
+   [
+    "P.4A",
+    11
+   ],
+   [
+    "P.92",
+    14
+   ],
+   [
+    "P.20",
+    16
+   ]
+  ],
+  "P.21": [
+   [
+    "P.67",
+    1
+   ],
+   [
+    "P.75",
+    3
+   ],
+   [
+    "P.4A",
+    3
+   ],
+   [
+    "P.92",
+    6
+   ],
+   [
+    "P.20",
+    8
+   ]
+  ],
+  "P.67": [
+   [
+    "P.75",
+    5
+   ],
+   [
+    "P.4A",
+    5
+   ],
+   [
+    "P.92",
+    8
+   ],
+   [
+    "P.20",
+    10
+   ]
+  ],
+  "P.75": [
+   [
+    "P.92",
+    3
+   ],
+   [
+    "P.20",
+    5
+   ]
+  ],
+  "P.4A": [
+   [
+    "P.92",
+    3
+   ],
+   [
+    "P.20",
+    5
+   ]
+  ],
+  "P.92": [
+   [
+    "P.20",
+    2
+   ]
+  ],
+  "P.20": [],
+  "P.5": [
+   [
+    "P.1",
+    12
+   ],
+   [
+    "P.103",
+    13
+   ]
+  ],
+  "P.81": [
+   [
+    "P.1",
+    4
+   ],
+   [
+    "P.103",
+    5
+   ]
+  ],
+  "P.82": [],
+  "P.84": [],
+  "P.87": [],
+  "P.77": [],
+  "P.85": [],
+  "P.76": []
+ },
+ "dam_stations": [
+  "P.1",
+  "P.103",
+  "P.21",
+  "P.5",
+  "P.67",
+  "P.81"
+ ],
+ "monsoon_months": [
+  6,
+  7,
+  8,
+  9,
+  10
+ ],
+ "ffill_limit_h": 3,
+ "basin_anchor": "P.1",
+ "forward_rain_h": 24,
+ "horizons": [
+  6,
+  12,
+  24
+ ],
+ "stale_after_h": 6.0,
+ "heuristic_sigma": 0.3,
+ "heuristic_version": "heuristic-v1",
+ "latest_window_h": 336,
+ "hii_fill_stations": [
+  "P.1",
+  "P.103",
+  "P.20",
+  "P.4A",
+  "P.67",
+  "P.75",
+  "P.82",
+  "P.84",
+  "P.92",
+  "P.81"
+ ],
+ "hii_exact_mirrors": [
+  "P.1",
+  "P.103",
+  "P.20",
+  "P.4A",
+  "P.67",
+  "P.75",
+  "P.82",
+  "P.84",
+  "P.92"
+ ],
+ "hii_min_overlap_hours": 168,
+ "dam": {
+  "dam_id": "200103",
+  "report_hour": 7,
+  "ffill_limit_h": 48,
+  "serving_days": 21
+ },
+ "notify": {
+  "clear_margin_m": 0.1,
+  "capacity_guard_min_pct": 60.0,
+  "capacity_guard_exempt": [
+   "P.1"
+  ],
+  "outlook_on": 0.5,
+  "outlook_off": 0.25,
+  "outlook_horizon": 24,
+  "feed_stale_after_h": 3.0,
+  "station_names": {
+   "P.1": "Nawarat Bridge, Chiang Mai city",
+   "P.103": "Ring Road Bridge 3, Chiang Mai",
+   "P.67": "Ban Tae (Mae Taeng)",
+   "P.21": "Ban Rim Tai (Mae Rim)",
+   "P.75": "Ban Chai Lat",
+   "P.92": "Ban Muang Aut",
+   "P.20": "Ban Chiang Dao",
+   "P.4A": "Ban Mae Taeng",
+   "P.5": "Tha Nang Bridge (downstream)",
+   "P.81": "Ban Pong (downstream)",
+   "P.82": "Ban Sob Win",
+   "P.84": "Ban Panton",
+   "P.87": "Ban Pa Sang",
+   "P.77": "Ban Sop Mae Sapuat",
+   "P.85": "Ban Lai Kaew",
+   "P.76": "Ban Mae I Hai"
+  }
+ }
+}