--fa4_prefill now accepts three states: True (force on, warn + fall
back on sub-Hopper), False (force off), None / default (auto-enable
where supported). Argparse switches to BooleanOptionalAction so both
--fa4_prefill and --no-fa4_prefill work, with the default being
auto-detect from torch.cuda.get_device_capability.
Adds a cu13 / cu12 install section and a per-GPU support matrix to
scripts/benchmarks/README.md.
Adds tests/test_fa4_capability_guard.py covering the nine
combinations of (explicit-on / auto / explicit-off) x (sm_80 / sm_90
/ sm_100 / sm_120). Monkey-patches get_device_capability and stubs
PageTable / patch_qwen3_model so it runs without CUDA.
