Extend primary_heartbeat_fn to read last_push_time per standby and send NOP
only when elapsed >= heartbeat_interval_s. on_standby_connected writes
last_push_time before ctrlhello send. Remove workaround xce_nop_req at end
of on_standby_connected. Add ha_nop_req_hook XCE_TESTING seam. 3 new tests.
Remove hb_vtask_t, hb_timeout_thread, s_hb_vtask from ha.c. Replace with
liveness_poll_ctx_t + delaymgr liveness_poll_fn: checks cancelled flag and
role==CTRL_STANDBY before acting; re-arms at heartbeat_interval_ms/2.
ha_on_primary_ctrl_fd_rdhup calls ha_disarm_heartbeat_timeout_vtask first
to prevent duplicate failover (Req 5.4). ha_on_heartbeat_received writes
last_activity via clock_gettime. Add test_ha_liveness.c with 7 unit tests.
