OpenFollow 0.3.1 RC3

Prerelease of 0.3.1 for testing.

Headline change since RC2: Enhanced mouse input (#10)

• Grab-by-ground-circle control – left-click a marker's ground circle to take control, right-click to release; double right-click resets the marker to its default position.
• macOS support – mouse input now works on macOS via per-frame GDK pointer polling (previously dead there).
• Steering refinements – pixel hysteresis deadband, smoothing (per-frame EMA glide), an upstage Y+ cap, and configurable scroll-wheel height control.
• Lens-distortion aware – clicks are undistorted before unprojection and the grab hit-test follows the bowed ground circle, so the clickable region matches what is drawn.

Review fixes folded in

• Scroll wheel keeps working on discrete-only devices (no smooth-scroll deltas) without double-counting notches.
• The double-right-click reset can no longer fire across two separate grab/release cycles.
• The per-frame mouse-glide and pointer-poll error logs are rate-limited.

The installable .deb / .ofupdate bundle and the Pi 5 / CM5 OS images are attached automatically once the release build finishes.