fix(compare): stale basket fetch no longer blanks a freshly opened comparison

Opening a compare link while localStorage held a different basket raced a
fetch for the OLD school set against the URL's SSR data; the stale
response replaced comparisonData, so no active school had data and every
section (including the trends chart) vanished until a hard refresh.
Responses from superseded effect runs are now dropped, successful ones
merge instead of replace, and the URL-seed effect re-runs when a
client-side navigation changes ?urns=.

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

nextjs-app/components/ComparisonView.tsx

@@ -85,7 +85,9 @@ export function ComparisonView({
         replaceSchools(urlSchools);
       }
     }
-  }, [isInitialized]); // eslint-disable-line react-hooks/exhaustive-deps
+    // Re-seed when a client-side navigation lands on a different ?urns= set
+    // (initialUrns/initialData are new props on the same component instance).
+  }, [isInitialized, initialUrns.join(',')]); // eslint-disable-line react-hooks/exhaustive-deps
 
   const urnKey = selectedSchools.map((s) => s.urn).join(',');
 
@@ -128,9 +130,18 @@ export function ComparisonView({
     const covered = urnKey.split(',').every((urn) => have[urn] != null);
     if (covered) return;
 
+    // Guard against out-of-order responses: while the basket hydrates from
+    // localStorage it can transiently hold a DIFFERENT school set than the
+    // URL, firing a fetch for schools the user is no longer comparing. That
+    // stale response must not replace data for the current set — replacing
+    // it blanked every section until a hard refresh. We (a) drop responses
+    // from superseded effect runs and (b) merge rather than replace, so data
+    // for the current schools always survives.
+    let cancelled = false;
     fetchComparison(urnKey, { cache: 'no-store' })
       .then((data) => {
-        setComparisonData(data.comparison);
+        if (cancelled) return;
+        setComparisonData((prev) => ({ ...(prev ?? {}), ...data.comparison }));
         setNationalAverages(data.national_averages);
         setBenchmarks(data.benchmarks);
       })
@@ -140,6 +151,9 @@ export function ComparisonView({
         // destroy a working comparison the user is looking at.
         console.error('Failed to fetch comparison:', err);
       });
+    return () => {
+      cancelled = true;
+    };
   }, [urnKey, isInitialized]);
 
   const primarySchools = selectedSchools.filter((school) => {