fix: smooth the quadrant trail and fade it by recency

The single solid trail line read like a tangle. Make older→newer legible: the
path now fades from muted slate (older) to bright blue (newer) via per-point
colors, the connecting line is faint, and the points are de-noised with a
centered moving average (today kept exact). Easier to see the direction of travel
through the quadrants.

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

frontend/src/components/regime/RegimeQuadrant.tsx

@@ -3,6 +3,7 @@ import { useQuery } from '@tanstack/react-query';
 import {
   ScatterChart,
   Scatter,
+  Cell,
   XAxis,
   YAxis,
   ZAxis,
@@ -29,6 +30,33 @@ interface QPoint {
   date: string;
 }
 
+/** Centered moving average to de-noise the path; today (last) kept exact. */
+function smoothTrail(points: QPoint[], half = 2): QPoint[] {
+  const n = points.length;
+  return points.map((p, i) => {
+    if (i === n - 1) return { ...p };
+    let sx = 0;
+    let sy = 0;
+    let c = 0;
+    for (let j = Math.max(0, i - half); j <= Math.min(n - 1, i + half); j++) {
+      sx += points[j].x;
+      sy += points[j].y;
+      c += 1;
+    }
+    return { x: sx / c, y: sy / c, date: p.date };
+  });
+}
+
+/** Recency gradient: 0 = oldest (muted slate), 1 = newest (bright blue). */
+function recencyColor(t: number): string {
+  const lerp = (a: number, b: number) => Math.round(a + (b - a) * t);
+  const r = lerp(71, 96);
+  const g = lerp(85, 165);
+  const b = lerp(105, 250);
+  const alpha = (0.3 + 0.7 * t).toFixed(2);
+  return `rgba(${r}, ${g}, ${b}, ${alpha})`;
+}
+
 function QuadrantTip({ active, payload }: { active?: boolean; payload?: { payload: QPoint }[] }) {
   if (!active || !payload?.length) return null;
   const p = payload[0].payload;
@@ -54,6 +82,7 @@ export default function RegimeQuadrant() {
       .map((p) => ({ x: p.index, y: p.early_warning as number, date: p.date }));
   }, [history.data]);
 
+  const trail = useMemo(() => smoothTrail(points), [points]);
   const latest = points.length ? points[points.length - 1] : null;
 
   return (
@@ -110,10 +139,18 @@ export default function RegimeQuadrant() {
                   axisLine={false}
                   label={{ value: 'Early warning', angle: -90, position: 'insideLeft', fill: '#6b7280', fontSize: 10 }}
                 />
-                <ZAxis range={[16, 16]} />
+                <ZAxis range={[13, 13]} />
                 <Tooltip cursor={{ strokeDasharray: '3 3', stroke: 'rgba(255,255,255,0.2)' }} content={<QuadrantTip />} />
-                {/* Trail (chronological path) */}
-                <Scatter data={points} fill="#60a5fa" fillOpacity={0.5} line={{ stroke: 'rgba(96,165,250,0.3)' }} isAnimationActive={false} />
+                {/* Smoothed trail with a recency gradient (old → new) */}
+                <Scatter
+                  data={trail}
+                  line={{ stroke: 'rgba(96,165,250,0.18)', strokeWidth: 1.5 }}
+                  isAnimationActive={false}
+                >
+                  {trail.map((_, i) => (
+                    <Cell key={i} fill={recencyColor(trail.length <= 1 ? 1 : i / (trail.length - 1))} />
+                  ))}
+                </Scatter>
                 {/* Today */}
                 {latest && (
                   <Scatter
@@ -135,9 +172,9 @@ export default function RegimeQuadrant() {
             <span><span className="text-red-400">④ Real downturn</span> — regime breaking, broad</span>
           </div>
           <p className="mt-2 text-[11px] leading-relaxed text-gray-600">
-            White dot = today; trail = path over the last {TRAIL} sessions. The tell isn&apos;t a single spot but the
-            move ①→④ (early warning rolling over while the regime index climbs = divergence resolving downward).
-            Observational — not wired into trades.
+            White dot = today; the trail fades from muted (older) to bright blue (newer) over the last {TRAIL}{' '}
+            sessions, smoothed. The tell isn&apos;t a single spot but the move ①→④ (early warning rolling over while
+            the regime index climbs = divergence resolving downward). Observational — not wired into trades.
           </p>
         </>
       )}