perf(compare): fetch only on school-set changes; use SSR payload; parallel page fetches

- Metric changes no longer refire /api/compare (the data is already
  client-side; the picker is presentational) — the fetch effect depends
  only on the URN set, with URL sync split into its own effect.
- The initial client fetch is skipped when the SSR payload already covers
  the selected schools; national averages + benchmarks now arrive via SSR
  props so nothing is lost by skipping.
- page.tsx fetches comparison and metrics in parallel.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0146VHeLAWjDVE2B5uU67jCB

nextjs-app/app/compare/page.tsx

@@ -32,26 +32,24 @@ export default async function ComparePage({ searchParams }: ComparePageProps) {
   const selectedMetric = metricParam || 'rwm_expected_pct';
 
   try {
-    // Fetch comparison data if URNs provided
-    let comparisonData = null;
-    if (urns.length > 0) {
-      try {
-        const response = await fetchComparison(urnsParam!);
-        comparisonData = response.comparison;
-      } catch (error) {
-        console.error('Failed to fetch comparison:', error);
-      }
-    }
+    // Fetch comparison + metrics in parallel — they are independent.
+    const [comparisonResponse, metricsResponse] = await Promise.all([
+      urns.length > 0
+        ? fetchComparison(urnsParam!).catch((error) => {
+            console.error('Failed to fetch comparison:', error);
+            return null;
+          })
+        : Promise.resolve(null),
+      fetchMetrics(),
+    ]);
 
-    // Fetch available metrics
-    const metricsResponse = await fetchMetrics();
-
-    // Metrics is already an array
     const metricsArray = metricsResponse?.metrics || [];
 
     return (
       <ComparisonView
-        initialData={comparisonData}
+        initialData={comparisonResponse?.comparison ?? null}
+        initialNationalAverages={comparisonResponse?.national_averages}
+        initialBenchmarks={comparisonResponse?.benchmarks}
         initialUrns={urns}
         metrics={metricsArray}
         selectedMetric={selectedMetric}