Add measurement and locating tools

## Summary
Allowing measuring distances and angles on the plot, and marking/tracking compass orientations and GNSS positions when available.

## Additional information
- [ ] hovering should show the distance and angle from the sensor front
- [ ] and the global position if there's a position estimate available through the MAVLink telemetry or an external NMEA system (per #518)
- click to add(/remove) angles/shapes/points of interest
- [ ] angle only makes sense if vehicle telemetry provides compass heading
- [ ] a shape (polygon/circle) of interest could be used for lining up something like a dock pile and maintaining position relative to it while inspecting it
- [ ] if there is a position estimate, can move points around on the display as the vehicle moves, and change them into arrows once they're outside the scan range
- [ ] click and drag to select a linear, rectangular, or pie slice region on the plot, with corresponding measurements included (e.g. lengths/angles/areas (per #515)
- #999 could make some of these more useful, by making the estimates horizontal instead of sensor-plane-aligned

The issue names no implementation files, tests, or entry points. Start by clarifying the plot and telemetry entry points and which measurement subset is in scope; done would need defined behavior for hover, annotations, region selection, and available-position or heading handling.

Written by the indexing model from the issue text.