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.

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