feat: adaptive trail sampling for smoother ribbons with less geometry

[MikePehel]

Summary

• Replace fixed-interval trail sampling (every 16ms) with direction-aware adaptive sampling that records points densely on turns and sparsely on straights
• Halve trail buffer from 3600 to 1800 points per vehicle since adaptive sampling achieves comparable coverage with fewer points
• Restore Snow view mode speed ribbon gradient (navy → blue → teal → green → yellow → red) which was falling through to the Grid default

How it works

Three sampling triggers, whichever fires first:

• Direction change: dot product < 0.995 (~5.7° turn) with minimum distance gate — curves get dense point coverage
• Max distance: 0.5m cap so straight runs don't gap
• Time floor: 64ms fallback for slow movement / hover

Impact

• Smoother ribbon geometry on turns (more points where curvature matters)
• ~50% less memory and draw cost per vehicle (1800 vs 3600 point buffer)
• Significant for multi-vehicle scenarios (16 drones = 28,800 fewer points per frame)
• No API changes, no new dependencies

Test plan

• Verify trail renders correctly in all three modes (off / directional / speed ribbon)
• Verify ribbon smoothness on tight turns vs straight runs
• Verify trail coverage duration is comparable to previous fixed-interval sampling
• Verify Snow mode speed ribbon uses distinct blue-green-yellow gradient
• Test with ULog replay (--replay)
• Test with live MAVLink input

[mrpollo]

Merged in #46