F3.1 KISS over serial: hardware TNCs on USB (TNC-X, NinoTNC) #177

Moved out of #120 so 0.4.0-alpha.1 can ship with KISS over TCP. Spec: docs/features/F3.1-kiss-sources.md behaviors 1.2, 4, 12; acceptance test 5.

• KISS over serial as a source kind: the port picked from the serial ports present (or typed), speed (default 9600), TNC port under "More". On APRS.NET's KissSerialConnection / KissSerialOptions (0.3.0-alpha.2, dead-link fix in 0.3.0-alpha.3).
• Reopens after a USB unplug and replug, without a restart; the pill says so.
• GetSerialPortsAsync(): the engine machine's ports; on a gateway, operator only.
• Gateway: deployment notes for passing a USB serial device into the container (--device).

Test bed: a Kenwood TH-D72A's built-in TNC on USB serial (KISS mode; APRS.NET validated receive, transmit, unplug and a 30-minute soak with it in docs/kiss-validation.md), then the MFJ TNC-X and NinoTNC. Moved to 0.5.0-alpha.1 beside Direwolf and APRS-IS, since the library side is validated. APRS.NET 0.4.0-alpha.4 adds what this needs beyond the connection: an unplugged serial TNC is a lost link (tested with a TH-D72A on USB serial), KissConnection.Warning for a TNC sending text instead of KISS (shown on the pill since #239), and QuietWarning for a radio switched off behind a port that stays open (F3.1 behavior 13, decision 5: 30 minutes, serial only).