feat: replay shows when the model would have detected and warned

The 2024 snapshot now carries a model track: p_warning and predicted
24 h peak from HGB heads trained ONLY on pre-flood data (an honest
backtest, cutoff 2024-08-31), plus its calibration sigma. During replay:

- the map clock shows the system's state at each moment: 'model: no
  flood expected' -> 'elevated risk' -> amber 'MODEL ALERT - flooding
  within 24 h likely (predicted peak X m)' -> red 'FLOODING' once the
  river actually crosses stage 1. First alert fires 24 Sep 01:00, a
  full day before the water crossed the flood line on 25 Sep.
- the flood-risk outlook stage chips re-render each frame from the
  historic model prediction (sigmoid over the stage levels), so the
  whole outlook panel time-travels with the map.

src/static/dashboard.html

@@ -137,6 +137,10 @@
         }
         .replay-clock strong { display: block; font-size: 1.65rem; font-weight: 800; letter-spacing: -.02em; white-space: nowrap; }
         .replay-clock span { font-size: .78rem; opacity: .85; }
+        .replay-alert { display: block; margin-top: 5px; font-size: .82rem; font-weight: 800; min-height: 1.1em; }
+        .replay-alert.note { color: #9fc9e8; }
+        .replay-alert.warn { color: #f5c542; }
+        .replay-alert.flood { color: #ff8a7a; }
         .forecast-grid { display: grid; grid-template-columns: repeat(auto-fill, minmax(215px, 1fr)); gap: 10px; margin-top: 14px; }
         .forecast-station { border: 1px solid var(--border); border-radius: 12px; padding: 10px 12px; }
         .forecast-station strong { font-size: .8rem; }
@@ -221,7 +225,7 @@
             </div>
             <div class="loading-panel" id="loading"><div class="loading-card">Loading river conditions…</div></div>
             <div class="error-panel" id="error"><div><strong>Map data could not be loaded.</strong><br><span id="error-message"></span></div></div>
-            <div class="replay-clock" id="replay-clock"><strong id="replay-clock-time">—</strong><span id="replay-clock-sub"></span></div>
+            <div class="replay-clock" id="replay-clock"><strong id="replay-clock-time">—</strong><span id="replay-clock-sub"></span><span class="replay-alert" id="replay-alert"></span></div>
         </article>
 
         <aside class="side-card">
@@ -833,6 +837,39 @@
                 $('p1-peak').textContent = `⏪ 2024 flood replay · ${when} · P.1 at ${p1Text} · basin flow ${Math.round(totalFlow).toLocaleString()} m³/s`;
                 $('replay-clock-time').textContent = ts.toLocaleString([], { day: 'numeric', month: 'short', year: 'numeric', hour: '2-digit', minute: '2-digit' });
                 $('replay-clock-sub').textContent = `P.1 ${p1Text} · basin flow ${Math.round(totalFlow).toLocaleString()} m³/s`;
+                // model track: what the forecast system (trained pre-flood) said at this moment
+                const model = data.model || {};
+                const pWarn = model.p_warn24 ? model.p_warn24[frame] : null;
+                const predMax = model.pred_max24 ? model.pred_max24[frame] : null;
+                // drive the flood-risk outlook chips from the historic model prediction
+                if (predMax != null && zoneData) {
+                    const sigma = model.sigma24 || 0.17;
+                    const stages = zoneData.features
+                        .map((f) => ({
+                            stage: f.properties.zone,
+                            level: Number(f.properties.level),
+                            p_exceed: 1 / (1 + Math.exp(-(predMax - f.properties.level) / sigma)),
+                        }))
+                        .filter((s, idx, arr) => arr.findIndex((o) => o.stage === s.stage) === idx)
+                        .sort((a, b) => a.stage - b.stage);
+                    state.p1Stages = stages;
+                    renderStageChips(stages, 24);
+                    $('p1-outlook').style.display = 'block';
+                }
+                const alertEl = $('replay-alert');
+                if (state.p1Now != null && state.p1Now >= 3.70) {
+                    alertEl.textContent = '🌊 FLOODING · river above stage 1 (3.70 m)';
+                    alertEl.className = 'replay-alert flood';
+                } else if (pWarn != null && pWarn >= 0.5) {
+                    alertEl.textContent = `⚠ MODEL ALERT · flooding within 24 h likely${predMax != null ? ` · predicted peak ${predMax.toFixed(2)} m` : ''}`;
+                    alertEl.className = 'replay-alert warn';
+                } else if (pWarn != null && pWarn >= 0.2) {
+                    alertEl.textContent = `model: elevated flood risk (${Math.round(pWarn * 100)}% within 24 h)`;
+                    alertEl.className = 'replay-alert note';
+                } else {
+                    alertEl.textContent = pWarn == null ? '' : 'model: no flood expected';
+                    alertEl.className = 'replay-alert note';
+                }
                 i += 2;                                 // 2 h per frame (~40 s total)
                 if (i >= n + 40) endReplay();           // hold the last frame briefly
             }, 120);
@@ -875,6 +912,19 @@
         return '#1e8b60';
     }
 
+    function renderStageChips(stages, horizonHours) {
+        const strip = $('p1-stages');
+        strip.replaceChildren();
+        (stages || []).forEach((s) => {
+            const chip = document.createElement('div');
+            chip.className = 'stage-chip';
+            chip.style.background = stageColor(s.p_exceed);
+            chip.title = `Stage ${s.stage}: river at ${s.level.toFixed(2)} m — ${Math.round(s.p_exceed * 100)}% within ${horizonHours} h`;
+            chip.innerHTML = `${Math.round(s.p_exceed * 100)}%<small>S${s.stage} · ${s.level.toFixed(2)} m</small>`;
+            strip.appendChild(chip);
+        });
+    }
+
     function renderP1Outlook(rows) {
         const card = $('p1-outlook');
         const p1rows = rows.filter((r) => r.station_code === 'P.1' && Array.isArray(r.stages));
@@ -902,16 +952,7 @@
         restyleFloodZones();
         $('p1-peak').textContent = (simulated ? `⚠ SIMULATION: P.1 at ${demoLevel.toFixed(2)} m (real: ${Number(row.current_level).toFixed(2)} m)` :
             `Now ${Number(row.current_level).toFixed(2)} m`) + ` · predicted peak next ${row.horizon_hours} h: ${Number(row.predicted_max_level).toFixed(2)} m`;
-        const strip = $('p1-stages');
-        strip.replaceChildren();
-        row.stages.forEach((s) => {
-            const chip = document.createElement('div');
-            chip.className = 'stage-chip';
-            chip.style.background = stageColor(s.p_exceed);
-            chip.title = `Stage ${s.stage}: river at ${s.level.toFixed(2)} m — ${Math.round(s.p_exceed * 100)}% within ${row.horizon_hours} h`;
-            chip.innerHTML = `${Math.round(s.p_exceed * 100)}%<small>S${s.stage} · ${s.level.toFixed(2)} m</small>`;
-            strip.appendChild(chip);
-        });
+        renderStageChips(row.stages, row.horizon_hours);
         card.style.display = 'block';
     }