Function gqa_pack_enabled 

pub fn gqa_pack_enabled() -> bool

The armed/disarmed answer for this process, resolved ONCE.

OnceLock, not a per-call env::var: this is read on the decode path, per layer per token, and spark_model’s hot_path_env_guards exists because that read takes the process-wide environment lock and costs 5.76 us at 16 threads. Resolving once is also what CUDA graph capture requires — the kernel a captured graph holds cannot depend on an environment read that might answer differently on replay.