SkySim - HITL

Building the Hardware-in-the-loop is the newest version of the SkySim drone simulator. SkySim can now drive a physical PX4 flight controller over USB (CDC-ACM), not just SITL. New SerialTransport + ITransport abstraction, a heartbeat-based autopilot probe, and a link router that auto-selects HITL vs SITL. Enable via the SITLManager inspector (enabled_hitl, serial_port, serial_baud). ArduPilot boards are detected and redirected to SITL (ArduPilot has no HITL). Existing SITL paths unchanged. See docs/hitl_bridge.md.
Requires the PX4 board in HITL mode (SYS_HITL=1 + HIL airframe) — one-time QGroundControl setup.