LOW 1: sel_t_buf CudaSlice + per-step memcpy_htod → sel_t_pinned
device-mapped. Zero copies, GPU reads directly from host memory.
LOW 2: is_q_gap_frozen uniform [q_gap; 4] → actual per-branch Q-gaps
from 48B DtoH readback in reduce_current_q_stats. Each branch now
independently detected as frozen/unfrozen. Liquid tau and trajectory
backtracking use real per-branch velocities.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>