Add comparison stats: failure rate %, vs-yesterday change, today total

Stats endpoint now returns current + previous period (24h shift) values
plus today's total count. UI shows:
- Total Matches: "of 12.3K today"
- Avg Duration: arrow + % vs yesterday
- Failure Rate: percentage of errors vs total, arrow + % vs yesterday
- P99 Latency: arrow + % vs yesterday
- In-Flight: unchanged (running executions)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/pages/executions/ExecutionExplorer.tsx

@@ -6,6 +6,20 @@ import { SearchFilters } from './SearchFilters';
 import { ResultsTable } from './ResultsTable';
 import styles from './ExecutionExplorer.module.css';
 
+function formatCompact(n: number): string {
+  if (n >= 1_000_000) return `${(n / 1_000_000).toFixed(1)}M`;
+  if (n >= 1_000) return `${(n / 1_000).toFixed(1)}K`;
+  return n.toLocaleString();
+}
+
+function pctChange(current: number, previous: number): { text: string; direction: 'up' | 'down' | 'neutral' } {
+  if (previous === 0) return { text: 'no prior data', direction: 'neutral' };
+  const pct = ((current - previous) / previous) * 100;
+  if (Math.abs(pct) < 0.5) return { text: '~0% vs yesterday', direction: 'neutral' };
+  const arrow = pct > 0 ? '\u2191' : '\u2193';
+  return { text: `${arrow} ${Math.abs(pct).toFixed(1)}% vs yesterday`, direction: pct > 0 ? 'up' : 'down' };
+}
+
 export function ExecutionExplorer() {
   const { toSearchRequest, offset, limit, setOffset, live, toggleLive } = useExecutionSearch();
   const searchRequest = toSearchRequest();
@@ -24,6 +38,17 @@ export function ExecutionExplorer() {
   const total = data?.total ?? 0;
   const results = data?.data ?? [];
 
+  // Failure rate as percentage
+  const failureRate = stats && stats.totalCount > 0
+    ? (stats.failedCount / stats.totalCount) * 100 : 0;
+  const prevFailureRate = stats && stats.prevTotalCount > 0
+    ? (stats.prevFailedCount / stats.prevTotalCount) * 100 : 0;
+
+  // Comparison vs yesterday
+  const avgChange = stats ? pctChange(stats.avgDurationMs, stats.prevAvgDurationMs) : null;
+  const failRateChange = stats ? pctChange(failureRate, prevFailureRate) : null;
+  const p99Change = stats ? pctChange(stats.p99LatencyMs, stats.prevP99LatencyMs) : null;
+
   const showFrom = total > 0 ? offset + 1 : 0;
   const showTo = Math.min(offset + limit, total);
 
@@ -43,10 +68,10 @@ export function ExecutionExplorer() {
 
       {/* Stats Bar */}
       <div className={styles.statsBar}>
-        <StatCard label="Total Matches" value={total.toLocaleString()} accent="amber" change={`from current search`} sparkData={sparkTotal} />
-        <StatCard label="Avg Duration" value={stats ? `${stats.avgDurationMs.toLocaleString()}ms` : '--'} accent="cyan" change="mean processing time" sparkData={sparkAvgDuration} />
-        <StatCard label="Failed" value={stats ? stats.failedCount.toLocaleString() : '--'} accent="rose" change="errored executions" sparkData={sparkFailed} />
-        <StatCard label="P99 Latency" value={stats ? `${stats.p99LatencyMs.toLocaleString()}ms` : '--'} accent="green" change="99th percentile" sparkData={sparkP99} />
+        <StatCard label="Total Matches" value={total.toLocaleString()} accent="amber" change={stats ? `of ${formatCompact(stats.totalToday)} today` : 'from current search'} sparkData={sparkTotal} />
+        <StatCard label="Avg Duration" value={stats ? `${stats.avgDurationMs.toLocaleString()}ms` : '--'} accent="cyan" change={avgChange?.text} changeDirection={avgChange?.direction} sparkData={sparkAvgDuration} />
+        <StatCard label="Failure Rate" value={stats ? `${failureRate.toFixed(1)}%` : '--'} accent="rose" change={failRateChange?.text} changeDirection={failRateChange?.direction} sparkData={sparkFailed} />
+        <StatCard label="P99 Latency" value={stats ? `${stats.p99LatencyMs.toLocaleString()}ms` : '--'} accent="green" change={p99Change?.text} changeDirection={p99Change?.direction} sparkData={sparkP99} />
         <StatCard label="In-Flight" value={stats ? stats.activeCount.toLocaleString() : '--'} accent="blue" change="running executions" sparkData={sparkActive} />
       </div>