onHandshakeComplete

Synchronously cancels the transport-aware handshake watchdog the moment the firmware signals Stage 2 completion (NODE_INFO_NONCE received).

This MUST be invoked before the asynchronous NodeDB install work begins so that a slow DB commit on a large mesh cannot trip the 12 s fast-recovery timeout after the firmware handshake has already succeeded. The watchdog cancellation it performs is a strict subset of onNodeDbReady; the remaining post-NodeDB side effects (analytics, MQTT start, history replay, telemetry requests) stay gated on onNodeDbReady at the end of the DB install block.