Doc-only: records that the http-listen 'handler-mutex deadlock' is not a
mutex bug but an Erlang-scheduler-context issue (handler runs on a native
Thread.create outside any er-sched step, so gen_server:call->receive can
never complete). Pattern A inapplicable; correct fix is Pattern B in
er-bif-http-listen (lib/erlang, m2 scope). Full diagnosis + patch sketch in
plans/fed-sx-host-primitives.md.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>