The documented CUDA-MTP opt-in was silently inert through the wrapper: _auto_tune exported DRAFT=0 (compute-bound, or disk-bound with projected hit < 0.90), which preempts the engine's DRAFT=-1 auto path - the only place COLI_CUDA_MTP is consulted (colibri.c). Found while running the #467 A/B: COLI_CUDA_MTP=1 alone measured pure baseline noise until DRAFT=3 was also set. Now an explicit COLI_CUDA_MTP=1 makes the planner leave DRAFT unset (engine resolves draft=3); unset keeps DRAFT=0 -> MTP off under CUDA, the measured-correct default (#389 -42 streaming-bound). ENVIRONMENT.md documents the behaviour and the measured trade-off.
Mohamed Mastouri
ac04b37af9