use async delivery for buffered performance observer entries

Per spec, buffered entries should be delivered via a queued task,
not synchronously. Extract scheduling logic into
schedulePerformanceObserverDelivery() and use it from both
notifyPerformanceObservers and the buffered observe path.

src/browser/Page.zig

@@ -1243,6 +1243,11 @@ pub fn notifyPerformanceObservers(self: *Page, entry: *Performance.Entry) !void
         }
     }
 
+    try self.schedulePerformanceObserverDelivery();
+}
+
+/// Schedules async delivery of buffered performance observer records.
+pub fn schedulePerformanceObserverDelivery(self: *Page) !void {
     // Already scheduled.
     if (self._performance_delivery_scheduled) {
         return;

src/browser/webapi/PerformanceObserver.zig

@@ -116,6 +116,7 @@ pub fn observe(
 
     // Deliver existing entries if buffered option is set.
     // Per spec, buffered is only valid with the type option, not entryTypes.
+    // Delivery is async via a queued task, not synchronous.
     if (options.buffered and options.type != null) {
         for (page.window._performance._entries.items) |entry| {
             if (self.interested(entry)) {
@@ -123,7 +124,7 @@ pub fn observe(
             }
         }
         if (self.hasRecords()) {
-            try self.dispatch(page);
+            try page.schedulePerformanceObserverDelivery();
         }
     }
 }