fix(compare): drop the merge-on-fetch — cancellation alone fixes the race

Review finding: merging left a re-added school's old entry in the map
forever, so the covered check served stale data and the map grew
unboundedly. The cancelled flag already discards superseded responses,
which is the actual race fix; replacing keeps the map bounded to the
current selection and guarantees fresh refetches.

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

nextjs-app/components/ComparisonView.tsx

@@ -133,15 +133,16 @@ export function ComparisonView({
     // 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
+    // stale response must not replace data for the current set — it blanked
-    // it blanked every section until a hard refresh. We (a) drop responses
+    // every section until a hard refresh. Cleanup marks the run cancelled
-    // from superseded effect runs and (b) merge rather than replace, so data
+    // when urnKey moves on, so only the current selection's response is
-    // for the current schools always survives.
+    // applied (replacing the map keeps it bounded and guarantees a re-added
+    // school is refetched fresh rather than served a lingering old entry).
     let cancelled = false;
     fetchComparison(urnKey, { cache: 'no-store' })
       .then((data) => {
         if (cancelled) return;
-        setComparisonData((prev) => ({ ...(prev ?? {}), ...data.comparison }));
+        setComparisonData(data.comparison);
         setNationalAverages(data.national_averages);
         setBenchmarks(data.benchmarks);
       })