feat: add drone (ArduPilot SITL + Gazebo) simulation

[RadAlpaca11]

Summary

Adds a drone option to the sim CLI for flying an S500-class quadcopter
(ArduCopter SITL simulating a Pixhawk 2.4.8) in Gazebo Harmonic via the
ardupilot_gazebo plugin,
flown through QGroundControl instead of RViz/Joint GUI.

(Documentation also added)

• sim gazebo setup-drone — one-time build of ArduPilot SITL (pinned to
  Copter-4.3.7 to match real flight-controller firmware) and the
  ardupilot_gazebo plugin from source, kept separate from the Docker
  image build. Works around several environment gaps: Ubuntu 24.04 isn't
  supported by this ArduPilot release's own prereqs script, its vendored
  waf predates Python 3.12, and a couple of its C++ files are missing a
  standard header under GCC 13.
• gazebo/drone_description / gazebo/drone_bringup — hand-authored SDF
  world + launch file (the drone isn't generated via the OnShape
  pipeline like arm/chassis). Needs the Imu/NavSat/Sensors system
  plugins plus a <scene> element for the IMU sensor to actually
  produce data, and runs Gazebo's server+GUI in a single process
  (combined_gui) since a split-process server has no render context
  for its Sensors system.
• robots.json gains a type field to distinguish non-OnShape robots.
• SITL's MAVLink output targets the container's own default gateway
  (read from /proc/net/route at launch), since QGroundControl runs on
  the host and ArduPilot's serial-device parser can't resolve hostnames
  or listen for incoming connections.

---

• Also fixes two unrelated pre-existing bugs found along the way: a
  stray backslash in docker-compose-gpu.yml's venv setup command that
  broke the NVIDIA devcontainer's sim CLI install, and missing runtime
  dependencies (pypresence, trimesh, pyfqmr, requests) in
  pyproject.toml that only surfaced in an isolated venv rather than
  system/pixi Python.

Setup required (documented in docs/gazebo/drone.mdx)

• Run sim gazebo setup-drone once before the drone sim works
• Host firewall must allow inbound UDP 14550 (e.g. sudo ufw allow 14550/udp) -
  QGroundControl and SITL communicate directly over the host network

Test plan

• sim gazebo setup-drone builds ArduCopter SITL + the plugin
• sim gazebo drone renders the quadcopter in Gazebo
• ArduCopter SITL receives real IMU/position data from the plugin
• QGroundControl auto-connects and shows live telemetry
• Verify sim gazebo arm / sim gazebo chassis are unaffected (regression check)