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.

src/static/dashboard.html

@@ -870,7 +870,12 @@
                     alertEl.textContent = pWarn == null ? '' : 'model: no flood expected';
                     alertEl.className = 'replay-alert note';
                 }
-                i += 2;                                 // 2 h per frame (~40 s total)
+                // Variable speed: race through quiet days, slow to hour-by-hour while
+                // the model is detecting and the flood unfolds, so the alert sequence
+                // (elevated risk -> model alert -> flooding) is readable.
+                const critical = (pWarn != null && pWarn >= 0.5) || (state.p1Now != null && state.p1Now >= 3.5);
+                const elevated = (pWarn != null && pWarn >= 0.2) || (state.p1Now != null && state.p1Now >= 2.8);
+                i += critical ? 1 : elevated ? 2 : 4;
                 if (i >= n + 40) endReplay();           // hold the last frame briefly
             }, 120);
         } catch (error) {