feat: ULog replay enhancements — frame stepping, persistent trails, markers, and seek fix

[MikePehel]

Summary

• Pre-compute entire flight trail from ULog at init for instant trail restore on marker seek
• Add frame markers (B to drop, Shift+B to delete, [ ] to cycle with wrap-around)
• System markers auto-extracted from ULog flight mode changes
• Frame stepping with Shift+Arrow (20ms), Ctrl+Shift+Arrow (1s)
• Marker labels on timeline (diamonds for user markers, squares for system markers)
• Marker labels in viewport (spheres for user markers, cubes for system markers)
• Playhead ring colored to match current trail color
• Improve seek accuracy with wider index scan window and dead-reckoning for sparse position data
• Parameterize grid fog distance for per-mode tuning
• Update speed ribbon gradient

Test plan

• Replay a ULog file and verify trail persists when jumping between markers with [ ]
• Drop markers with B, verify they appear on timeline and in 3D
• Frame step with Shift+Arrow, confirm 20ms increments and auto-pause
• Verify system markers appear at flight mode transitions
• Wrap around from first to last marker and back — trail should persist
• Verify all view modes render correctly (Grid, Rez, Snow)
• Build on macOS and Linux to confirm cross-platform compatibility