ST2110NicStageChanged

Finer-grained NIC bring-up lifecycle. The pre-existing mtl_ready message in ST2110NicEvent already covers the most important "now ready" signal — this exposes the intermediate states the server transits between gen_server start and that point.

Sequence (per NIC):

INITIALISING — supervisor’s gen_server is up; pre-flight validation has passed; the heavy work is queued via a self-message LOADING_HOST — Api.createHost is mid-flight (spawning the isox plugin host OS process; zig’s load() runs inside) WAITING_MTL — host loaded; MTL_Init / DPDK / NIC ramp running inside the isox host process; awaiting completion READY — MTL is up and accepting per-session create calls; also fires the legacy mtl_ready event for back-compat FAILED — any phase rejected; reason carries the human message (same sanitised text as legacy mtl_init_failed)