fix(netio): bug-1447 same-batch endpoint use-after-free in io_worker dispatch

Raft-drive-async's reconnect-offload burst (Task 2.1/2.2) exposed a
latent race in io_worker's epoll dispatch loop: ha.c's
on_standby_connected() evicts a stale duplicate peer channel via
iomgr_unmonitor()+endpoint_unref(), but epoll_ctl(DEL) only stops
FUTURE epoll_wait() calls -- it does not invalidate an event for that
same endpoint already sitting in the CURRENT batch's events[] array.
A later event in the same batch then dereferences the freed
endpoint_t/SSL*, producing the ASAN SEGV observed deep inside
OpenSSL's ssl3_read_bytes during raft-drive-async's round-9 soak.

The original "concurrent SSL_do_handshake from two io_worker threads"
hypothesis was wrong and is retracted -- iomgr_start() creates exactly
one io_worker thread per iomgr_t.

Fix: extract io_worker_dispatch_batch(), which takes endpoint_ref() on
every endpoint in the batch before dispatching any of them and
endpoint_unref()s each only after the whole batch completes -- the
same lock-free ref/unref bracket already used for bug-1298 and
INVOKE_MSG_HANDLER, just never applied to io_worker's own loop. Pure
atomics, no locks, no blocking added to the hot path.

Adds test_netio_batch_dispatch_uaf.c, which drives the real dispatch
loop with a synthetic epoll_event[] batch (TDD RED confirmed via ASAN
heap-use-after-free before the fix, GREEN after). Verified further
with two independent clean 10-round live sequential HA soaks (33 and
36 jobs, zero loss) and full ASAN suite runs (xcelib_test 294/0,
controller_unit_test 972/0). raft-drive-async's kiro-validate-impl
re-run flips NO-GO to GO.

src/lib/meson.build

@@ -180,6 +180,7 @@ libxce_test_src = ['testsuite/test.c',
                    'testsuite/test_netio_arm_failure.c',
                    'testsuite/test_netio_close_propagation.c',
                    'testsuite/test_netio_bounded_connect.c',
+                   'testsuite/test_netio_batch_dispatch_uaf.c',
                    'testsuite/test_request_send_propagation.c',
                    'testsuite/test_raft.c',
                    'testsuite/test_wal_wire.c',