Add io_classify_io_error and propagate endpoint/epoll errors through
endpoint_update_epoll and cbuf_get instead of swallowing them, so client
failover surfaces the real error rather than hanging. Adds netio arm-failure,
close-propagation, and request send-propagation tests.
Pre-existing uncommitted work, committed to establish a clean baseline before
ha-commit-durability implementation.
Add void (*on_connected)(endpoint_t *ep) field to endpoint_t with
endpoint_set_on_connected() helper. io_handler fires it exactly once
immediately after ssl_handshake_done=1 then nulls the pointer.
Enables ha_start_election_async to send OP_requestVote as soon as
the TLS handshake with a peer completes — no blocking SSL_connect.
6 new unit tests pass; controller suite 1/1 OK.
