feat(mechanical): universal sensor mount rail system (Issue #138)
Add 2020 T-slot quick-swap sensor rail for SaltyLab/Rover/Tank variants:
- sensor_rail.scad: 2020 T-slot profile, T-nut, thumbscrew, end cap, index
pins, stem/post/tank clamp adapters
- sensor_rail_brackets.scad: universal T-nut base + RPLIDAR A1M8, D435i,
IMX219, UWB anchor, cable clip brackets (tool-free M3 thumbscrew retention)
- sensor_rail_BOM.md: purchased hardware, print settings, export commands
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>