chore: add happel.ai feed to benchmark suite

72MB feed with huge CDATA entries, good stress test for large feed handling.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: vprelovac

benchmark.py

@@ -214,6 +214,7 @@
     "https://frisk.space/index.xml",
     "https://www.youtube.com/feeds/videos.xml?channel_id=UCA7LqFpF_Zlty4Yg2iGJppw",
     "https://quartzlibrary.com/atom/",
+    "https://happel.ai/index.xml",
 ]
 
 HEADERS = {