feat: regime quadrant plot in place of the combined gauge

The combined score collapsed two distinct signals into one not-very-meaningful
number. Replace its gauge with a quadrant scatter that shows both axes directly:
x = regime index (coincident), y = early warning (breadth divergence), with a
trail of the last 60 sessions and today highlighted.

The four quadrants make the readings legible — ① hot & brittle (narrow melt-up,
shakeout risk), ② transition, ③ healthy & broad, ④ real downturn — and the trail
surfaces the actual tell: the ①→④ move (early warning rolling over as the regime
index climbs = divergence resolving downward). Combined still shows as a line in
the score-history chart. Frontend-only; reuses the history endpoint. Lazy-loaded.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/pages/RegimePage.tsx

@@ -18,6 +18,7 @@ import {
 
 // Lazy so recharts (heavy) ships in its own chunk, loaded only on this tab.
 const ScoreHistoryChart = lazy(() => import('../components/regime/ScoreHistoryChart'));
+const RegimeQuadrant = lazy(() => import('../components/regime/RegimeQuadrant'));
 import type {
   RegimeBand,
   RegimeSignal,
@@ -554,17 +555,9 @@ export default function RegimePage() {
               }
             />
           </div>
-          <ScoreGauge
-            label="Combined · observational blend"
-            score={monitor.data.combined?.score}
-            band={monitor.data.combined?.band}
-            trend={monitor.data.combined}
-            size="md"
-            footnote={
-              <>A weighted mean of the index and the early warning — for observation only. Tune the mix via the
-              regime config.</>
-            }
-          />
+          <Suspense fallback={<SkeletonCard className="h-80" />}>
+            <RegimeQuadrant />
+          </Suspense>
           <Suspense fallback={<SkeletonCard className="h-72" />}>
             <ScoreHistoryChart />
           </Suspense>