docs: render of actual vs predicted through the 2024 flood; adaptive replay speed

docs/img/backtest-2024-p1.png: observed P.1 level vs the 24h-ahead
predicted peak issued at each hour by a model trained only on pre-flood
data, with the warning-probability panel below (first alert 24 Sep
01:00, 24 h before flooding began). Embedded in FLOOD_FORECASTING.md's
headline-validation section with an honest reading, including the
~0.4 m peak under-prediction.

Replay pacing is now adaptive: 4 h/frame through quiet days, 2 h when
risk is elevated, 1 h (hour-by-hour) while the model is alerting or the
river is near/above flood stage - so viewers can watch the detection
sequence unfold.

docs/FLOOD_FORECASTING.md

@@ -291,6 +291,21 @@ training half. So the model was retrained on data **ending 2024-08-31** and aske
 to forecast September–November 2024 cold, with no knowledge of the event that
 followed. This is the closest thing to a real operational test available.
 
+![Observed P.1 level vs the model's 24 h-ahead predicted peak through the
+October 2024 flood, with the warning probability below](img/backtest-2024-p1.png)
+
+The render above shows the whole event hour by hour. Top: the observed level
+(blue) against the 24 h-ahead predicted peak the model issued at each hour
+(amber, dashed) — the amber line leads the blue one into both flood waves,
+which is the entire point of the system. Bottom: the model's probability of
+flooding within 24 h; it fires its **first alert at 01:00 on 24 September, a
+full day before the river crossed the 3.70 m flooding line**, stays pinned near
+1.0 through both waves, and stands down between and after them. Also visible,
+honestly: the predicted peak tops out ~0.4 m short of the actual 5.30 m record
+(the extreme-compression limitation discussed below), and the prediction is
+noisier on the recession limbs. The same model track drives the dashboard's
+"Replay Oct 2024 flood" feature, so this chart can be watched live on the map.
+
 - **25 September cold start.** P.1's first warning crossing of the episode was
   alerted **24–26 hours ahead**. This is the genuinely impressive case: the river
   was in normal state, and the alert came from upstream routing alone.