feat: recalibrate P.77/P.75 thresholds; capacity guard on alerts

The first ntfy cycle announced "Warning level at P.77" at 3.02 m. That
gauge's 2.85 m threshold sat below its own dry-season baseline (2.6-2.7 m
at 8-14 % channel capacity): P.77 had been "above warning" for 761 of the
last 2 146 hours, at 22 % capacity. Across 2018-2024, 75-85 % capacity
reads 3.35-4.57 m and 95-105 % reads 4.27-5.08 m; set 4.30 / 4.90. P.75
moved 2.75/3.50 -> 3.20/3.65 on the same evidence (2024: 3.45 / 3.72).
The predictor already handles changed thresholds (regression-derived
probabilities until the Oct 1 retrain).

Second line of defence in notify.py: a clear->alert transition is only
announced when RID's discharge_percent for the reading is >= 60 %, so a
re-rated or datum-shifted gauge cannot page subscribers again. P.1 is
exempt (its stages come from the inundation map, not capacity); readings
without a capacity figure fall back to level only; the all-clear edge is
never blocked. 3 tests.

src/ml/features.py

@@ -37,9 +37,17 @@ THRESHOLDS: Dict[str, Tuple[float, float]] = {
     "P.4A": (3.40, 3.90),
     "P.5": (4.55, 4.95),
     "P.67": (2.45, 2.90),
-    "P.75": (2.75, 3.50),
+    # P.75: 2024 (the only year with a full flood record, 191% capacity peak)
+    # puts 75-85% at 3.45 m and 95-105% at 3.72 m; 2018/2022 agree within
+    # 0.15 m. The 2026-08 value (2.75) alerted on 15 quiet-season hours.
+    "P.75": (3.20, 3.65),
     "P.76": (5.35, 5.45),
-    "P.77": (2.85, 3.35),
+    # P.77: recalibrated 2026-09-12. The 2026-08 value (2.85) sat below the
+    # gauge's own dry-season baseline (2.6-2.7 m at 8-14% capacity), so the
+    # first ntfy cycle fired a "warning" at 22% capacity. Across 2018-2024,
+    # 75-85% capacity reads 3.35-4.57 m and 95-105% 4.27-5.08 m; 2024 (the
+    # best-sampled flood year) gives 4.57 / 5.08. Slightly conservative:
+    "P.77": (4.30, 4.90),
     "P.81": (5.15, 6.30),
     # P.82 never reached 100% capacity in the record (max level 3.78, max 96.4%);
     # danger sits just below the observed maximum so the head can actually train.