Function prefill_bgmv_forced 

pub fn prefill_bgmv_forced() -> bool

2026-09-25: METRALE_LORA_PREFILL_BGMV=1, read once: the attention QKV prefill applies LoRA with the per-row apply_lora_bgmv instead of apply_lora_delta when the call has a slot buffer and a route (layers/qwen3_attention/prefill/paged_qkv.rs).