Hang up on a message the pod can't use instead of exiting

[ps2]

Pairing, session setup and the command loop exited the pod with log.Fatal when a message from the phone didn't parse, decrypt or unmarshal, and the packet layer exited on an unexpected command. In the Simulator nothing restarts the pod, so one bad message lost it. #20 hit this: an EAP-AKA Success that didn't parse.

• Pairing, session setup and commands: the pod logs the error, hangs up and waits for the phone to reconnect and set up a new session. This also covers a response ACK that doesn't decrypt or isn't empty, which used to exit the pod after the command had already been applied.
• Packet layer: an unexpected command is logged, and the pod carries on.
• Still exits: errors in the pod's own work, like generating keys, saving state or building a response.

Testing

• pkg/pod/pod_test.go sends a garbled message during session setup. On main it fails with the pod's EAP-AKA parse error; now the pod hangs up.
• go test -race ./... passes, and it builds for GOOS=linux GOARCH=arm.
• With Loop in the iOS Simulator, a session and a status read worked as before. The hang-up path is covered only by the unit test.