MAVLink crate

• Library rns_mavlink plus bins fc and gc
• Kaonic radio or UDP Reticulum interfaces (run-fc-udp.sh / run-gc-udp.sh vs run-*-kaonic.sh)
• Config: Fc.toml, Gc.toml
• Cross compile: build.sh / .cargo for armv7-unknown-linux-gnueabihf

Operator guide: operations/mavlink.