Initial commit: ArduSub 4.5.7 custom 6-motor ROV frame

Adds patch and build script for Pixhawk 2.4.8 with a non-standard
6-thruster configuration. Running build.sh clones the official
ArduPilot repo, applies the patch to AP_Motors6DOF.cpp (SUB_FRAME_CUSTOM),
and compiles the firmware automatically.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>