defhandler was only available via web-forms.sx which had load
dependencies that failed. Now registered as a native special form
in make_server_env, works in both coroutine and HTTP modes.
Key fix: custom special forms receive [List args; Env eval_env],
not flat args. The handler is now bound in the eval env, not the
make_server_env closure env.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>