F2.1 Own station: transmit enable and safety interlocks #190

Spec: to write first (docs/features/F2.1-….md), from the scope row.

The groundwork every transmit feature in 0.5.0–0.7.0 builds on:

• the station's callsign and SSID guidance, symbol, comment and a verified passcode in the profile;
• a Transmit switch, off by default, that can only turn on with a verified APRS-IS login (or, for RF, a licensed callsign), and is refused on N0CALL and when Engine__ReceiveOnly (F1.26) is set;
• every outgoing packet made through APRS.NET's PacketAddress with the TOCALL from Brand.Tocall (APZAMP until the registered one, see the 0.7.0 TOCALL issue), and sent through the connection's SendAsync;
• a sent-packets log in the packet list, marked as ours.

Gaps: G01 (PacketAddress, builders: delivered), G02 (send, passcode: delivered). No APRS.NET blocker.