feat(pure): pruneHistory with strict 30-day window

src/pure.js

@@ -50,3 +50,10 @@ export function computeEstimate(current, target, perTrain, perDay) {
 
   return { remaining, trainsToGo, days, eta };
 }
+
+const THIRTY_DAYS_MS = 30 * MS_PER_DAY;
+
+export function pruneHistory(entries, now = Date.now()) {
+  const cutoff = now - THIRTY_DAYS_MS;
+  return entries.filter((e) => e.ts > cutoff);
+}