solve velocity constraints for a specific island. only processes constraints at the given indices.
uses cached velocity locals to avoid repeated body property access during the solve loop.
for each constraint: velocities are loaded once, all contact point solves operate on locals,
then velocities are written back with DOF masking applied once.
solve velocity constraints for a specific island. only processes constraints at the given indices.
uses cached velocity locals to avoid repeated body property access during the solve loop. for each constraint: velocities are loaded once, all contact point solves operate on locals, then velocities are written back with DOF masking applied once.