fix: one current river level, not two

The verdict banner and the P.1 outlook each wrote state.p1Now from a
different feed — the banner from the latest measurement, the outlook from
current_level on the forecast rows, which carries whatever the model saw
at its as_of. Forecasts are precomputed hourly, so the two drifted apart:
production showed 1.66 m in the banner and 1.52 m in the outlook directly
below it. Harmless at low water; at flood stage two contradictory river
levels on one screen undermine the warning.

setP1Level() now arbitrates: freshest timestamp wins, and the replay and
demo hooks pass force since they deliberately pin a level that is not the
live one. The outlook renders the arbitrated value and clamps the shown
peak to at least the current level, so a stale forecast can no longer
predict a peak below where the river already is. endReplay drops the
replayed level so live data re-arbitrates cleanly.

Verified against a stub reproducing the exact production conditions
(gauge 1.66 at 08:35 vs forecast 1.52 at 08:00): both now read 1.66; a
2.50 m rise against a stale 1.81 m peak renders 2.50/2.50; the 2024
replay still tracks its frames and returns to live on stop.

src/static/dashboard.html

@@ -395,7 +395,7 @@
 <script>
 (function () {
     'use strict';
-    const state = { map: null, layers: [], markers: new Map(), hasFit: false, historyChart: null, selectedStation: null, historyRequestId: 0, p1Stages: null, p1Now: null, rainLayer: null, useDates: false, forecastExpanded: false };
+    const state = { map: null, layers: [], markers: new Map(), hasFit: false, historyChart: null, selectedStation: null, historyRequestId: 0, p1Stages: null, p1Now: null, p1NowAt: null, rainLayer: null, useDates: false, forecastExpanded: false };
     const $ = (id) => document.getElementById(id);
 
     // ---- Localisation -----------------------------------------------------
@@ -1328,6 +1328,30 @@
         });
     }
 
+    // Single source of truth for "the river right now".
+    //
+    // Two independent feeds carry a P.1 level: the latest measurement, and
+    // `current_level` on the forecast rows, which is whatever the model saw at
+    // its `as_of` (forecasts are precomputed hourly, so it lags). Both used to
+    // write state.p1Now unconditionally and the page showed two different
+    // "now" values at once — observed 1.66 m in the verdict banner against
+    // 1.52 m in the outlook directly below it. Harmless at low water, but at
+    // flood stage two contradictory river levels destroy trust in the warning.
+    //
+    // The freshest reading wins; `force` is for the replay and demo hooks,
+    // which deliberately pin a level that is not the live one.
+    function setP1Level(value, at, opts) {
+        const options = opts || {};
+        if (value == null || Number.isNaN(Number(value))) return;
+        const stamp = at ? new Date(at).getTime() : null;
+        const known = Number.isFinite(stamp) ? stamp : null;
+        if (!options.force && known != null && state.p1NowAt != null && known < state.p1NowAt) {
+            return; // an older snapshot must not overwrite a newer one
+        }
+        state.p1Now = Number(value);
+        if (known != null) state.p1NowAt = known;
+    }
+
     // At-a-glance verdict from P.1 level + 24 h forecast. Level thresholds are
     // the official Chiang Mai inundation stages (city flooding starts 3.70 m).
     function updateFloodVerdict() {
@@ -1368,7 +1392,7 @@
         // Flow at the basin anchor (summing sequential mainstem gauges would
         // double-count the same water, so no basin-wide total is shown).
         const p1 = readings.get('P.1');
-        if (p1?.water_level != null) state.p1Now = Number(p1.water_level);
+        setP1Level(p1?.water_level, p1?.timestamp);
         updateFloodVerdict();
         const p1Flow = p1?.discharge == null ? null : Number(p1.discharge);
         $('total-flow').textContent = p1Flow == null ? '—' : p1Flow.toLocaleString(loc(), { maximumFractionDigits: 1 });
@@ -1581,6 +1605,8 @@
     function endReplay() {
         if (state.replayTimer) window.clearInterval(state.replayTimer);
         state.replayTimer = null;
+        state.p1Now = null;      // drop the replayed level so live data re-arbitrates
+        state.p1NowAt = null;
         $('replay-2024').textContent = t('replay.start');
         $('replay-clock').style.display = 'none';
         setLiveIndicator('live');
@@ -1626,7 +1652,8 @@
                 $('peak-station').textContent = peakFlow ? t('replay.stress', peakFlow.code) : t('replay.nodischarge');
                 restyleRiver(readings);
                 const p1Level = data.stations['P.1'] ? data.stations['P.1'].level[frame] : null;
-                if (p1Level != null) state.p1Now = Number(p1Level);
+                // force: replay frames are 2024 timestamps, older than anything live
+                if (p1Level != null) setP1Level(p1Level, null, { force: true });
                 restyleFloodZones();
                 const ts = new Date(data.timestamps[frame]);
                 // minute included: a lone "17" reads as a year in Thai output
@@ -1736,31 +1763,38 @@
         if (!p1rows.length) { card.style.display = 'none'; return; }
         const row = p1rows.reduce((best, r) => r.horizon_hours > best.horizon_hours ? r : best);
         state.p1Stages = row.stages;
-        state.p1Now = row.current_level == null ? null : Number(row.current_level);
+        // Offered, not imposed: only used when it is fresher than the latest
+        // measurement (see setP1Level).
+        setP1Level(row.current_level, row.as_of);
         // Demo hooks: ?demo_level=4.4 pins a simulated P.1 level; ?demo_rise=1 animates
         // the water from the real level up to the 2024 record (5.30 m).
         const params = new URLSearchParams(location.search);
         const demoLevel = Number(params.get('demo_level'));
         const simulated = Number.isFinite(demoLevel) && demoLevel > 0;
-        if (simulated) state.p1Now = demoLevel;
+        if (simulated) setP1Level(demoLevel, null, { force: true });
         if ((simulated || params.has('demo_rise')) && !state.replayTimer) setLiveIndicator('sim', 'pill.sim');
         if (params.has('demo_rise') && !state.demoRiseTimer) {
             let level = state.p1Now ?? 2.2;
             state.demoRiseTimer = window.setInterval(() => {
                 level = Math.min(level + 0.05, 5.30);
-                state.p1Now = level;
+                setP1Level(level, null, { force: true });
                 $('p1-peak').textContent = t('outlook.sim.rising', level.toFixed(2), Number(row.current_level).toFixed(2));
                 restyleFloodZones();
                 if (level >= 5.30) window.clearInterval(state.demoRiseTimer);
             }, 700);
         }
         restyleFloodZones();
+        // Show the SAME level the verdict banner shows, and never let the
+        // predicted peak read below it (the peak was computed from the
+        // forecast's own, possibly older, current_level).
+        const shownNow = state.p1Now != null ? state.p1Now : Number(row.current_level);
+        const shownPeak = Math.max(Number(row.predicted_max_level), shownNow);
         $('p1-peak').textContent = simulated
             ? t('outlook.sim', demoLevel.toFixed(2), Number(row.current_level).toFixed(2))
-            : t('outlook.now', Number(row.current_level).toFixed(2), row.horizon_hours,
-                Number(row.predicted_max_level).toFixed(2));
+            : t('outlook.now', shownNow.toFixed(2), row.horizon_hours, shownPeak.toFixed(2));
         renderStageChips(row.stages, row.horizon_hours);
         card.style.display = 'block';
+        updateFloodVerdict(); // banner reflects the arbitrated level, not a stale one
     }
 
     async function loadForecasts() {

tests/test_dashboard.py

@@ -175,3 +175,25 @@ def test_dashboard_default_language_respects_browser_order():
 
     assert "langs.some" not in html  # the old any-English-wins test
     assert "if (code.startsWith('th')) return 'th';" in html
+
+
+def test_dashboard_shows_one_current_river_level():
+    """The verdict banner and the P.1 outlook must not disagree about "now".
+
+    Production served 1.66 m in the banner and 1.52 m in the outlook at the
+    same moment: the banner used the latest measurement, the outlook used the
+    forecast payload's current_level from an older as_of.
+    """
+    html = DASHBOARD_PATH.read_text(encoding="utf-8")
+
+    # One arbitrated writer, freshest-wins
+    assert "function setP1Level(" in html
+    assert "state.p1NowAt" in html
+
+    # Neither feed may assign the level directly any more
+    assert "state.p1Now = Number(p1.water_level)" not in html
+    assert "state.p1Now = row.current_level" not in html
+
+    # The outlook renders the arbitrated level, and never a peak below it
+    assert "const shownNow = state.p1Now != null" in html
+    assert "Math.max(Number(row.predicted_max_level), shownNow)" in html