ST2110NicSettings.onStageChange property

Called whenever the NIC’s bring-up advances to a new stage. Mirrors the proto ST2110NicStageChanged sequence: "initialising" → "loadingHost" → "waitingMtl" → "ready"; or "failed" with a reason if any phase rejects. The nic() promise still resolves on the legacy mtl_ready signal — this callback is purely for progress reporting (UI status, structured logs).

Signature:

onStageChange?: (event: ST2110NicStageEvent) => void;