Function keep_nvfp4_fallback 

pub fn keep_nvfp4_fallback() -> bool

2026-09-25: Whether METRALE_DENSE_FP8_KEEP_NVFP4 is set, which makes the plan build every NVFP4 copy even where the dispatch cannot reach it. Presence, like METRALE_FFN_W8A16_ONLY: any value, including empty and 0, turns it on. Read once per process.