examples: add simulated vision position feed

[alireza787b]

Summary

Adds a simulator-only example that feeds external-vision position data to PX4
through MAVSDK's Mocap plugin.

• reads the simulator's ground_truth() global position stream
• converts nearby WGS84 coordinates to a local NED frame
• applies configurable position noise and sample delay
• publishes VISION_POSITION_ESTIMATE at 30 Hz
• enables only horizontal and vertical external-position fusion
• monitors PX4 local-position validity without claiming that it proves fusion

The script does not arm, take off, change modes, or otherwise control the
vehicle. It saves and restores the existing EKF2_EV_CTRL value.

Measurement model

MAVSDK's simulator ground-truth stream provides independent position, but not
independent attitude or velocity. The example therefore sends position only.
It does not feed PX4's own EKF estimate back as a measurement, reinterpret NED
velocity as body-frame velocity, or enable yaw fusion without a yaw source.

VISION_POSITION_ESTIMATE is used because it is a documented PX4 NED input
that matches the measurements available from this source. A real VIO or motion
capture integration should replace the simulator source and provide every
enabled measurement in its documented frame.

Delay

DELAY_MS delays sample delivery. To evaluate EKF delay compensation, set
EKF2_EV_DELAY accordingly and reboot PX4 before running the example.

Validation

• ruff format --check --line-length=100 examples
• ruff check --select=ASYNC,RUF006,E,F --line-length=100 examples
• ruff check --select=PERF --line-length=100 .
• codespell .
• Python bytecode compilation
• WGS84-to-local-NED numerical check
• generated MAVSDK object and protobuf serialization check
• fake ground-truth stream integration check

The branch is rebased on current main and contains one commit.

[JonasVautherin]

Nice, thanks!

[alireza787b]

Thanks for the review Jonas! Just pushed a small update renaming quality → quality_percent to match the proto change from your feedback on #395.

[julianoes]

Good to be merged? The todos in the PR description aren't ticked, so I'm not so sure.

[alireza787b]

Hey Juliano, not yet. You are right that the description there was stale, thanks for flagging it. I just cleaned that up. This example is waiting on the fresh proto PR (MAVSDK-Proto#404) and then the clean replacement MAVSDK PR for the mocap wiring. After that I still want to do one final pass on the example itself before calling it merge-ready.

[alireza787b]

The upstream mocap dependencies have landed, so I completed the final validation pass and rebased this branch on current main.

That pass found two correctness problems in the earlier example: it used PX4's estimated position_velocity_ned() as the external measurement source, and it placed NED velocity in body-frame fields. The refreshed example now reads simulator ground_truth(), sends only the independent position data available from that source through VISION_POSITION_ESTIMATE, and restores the user's original EKF2_EV_CTRL value on exit.

This is a substantive rewrite, so the earlier approval should not be treated as covering the new revision. The branch is now one commit, the local style/serialization checks listed in the description pass, and fresh CI is running. A new review would be appreciated when it completes.