support buffered option in PerformanceObserver.observe()

When buffered is true, deliver existing performance entries that match
the observer's interest immediately, per the Performance Observer spec.

src/browser/tests/performance_observer/performance_observer.html

@@ -69,3 +69,31 @@
 <script>
   testing.expectEqual(['mark', 'measure'], PerformanceObserver.supportedEntryTypes);
 </script>
+
+<script id="buffered_option">
+{
+  // Create marks BEFORE the observer
+  performance.mark("early1", { startTime: 1.0 });
+  performance.mark("early2", { startTime: 2.0 });
+
+  let receivedEntries = null;
+  const observer = new PerformanceObserver((list) => {
+    receivedEntries = list.getEntries();
+  });
+
+  // With buffered: true, existing marks should be delivered immediately
+  observer.observe({ type: "mark", buffered: true });
+
+  testing.eventually(() => {
+    testing.expectEqual(true, receivedEntries !== null);
+    // Should have received at least the 2 early marks
+    testing.expectEqual(true, receivedEntries.length >= 2);
+
+    const names = receivedEntries.map(e => e.name);
+    testing.expectEqual(true, names.indexOf("early1") >= 0);
+    testing.expectEqual(true, names.indexOf("early2") >= 0);
+
+    observer.disconnect();
+  });
+}
+</script>

src/browser/webapi/PerformanceObserver.zig

@@ -113,6 +113,18 @@ pub fn observe(
 
     // Update interests.
     self._interests = interests;
+
+    // Deliver existing entries if buffered option is set.
+    if (options.buffered) {
+        for (page.window._performance._entries.items) |entry| {
+            if (self.interested(entry)) {
+                try self._entries.append(page.arena, entry);
+            }
+        }
+        if (self.hasRecords()) {
+            try self.dispatch(page);
+        }
+    }
 }
 
 pub fn disconnect(self: *PerformanceObserver, page: *Page) void {