Function resolve_policy 

pub fn resolve_policy(
    declared: &str,
    env_raw: Option<&str>,
) -> (SplitkPolicy, bool)

The target’s declaration, overridden by ATLAS_ATTN_DECODE_SPLITK.

Returns (policy, came_from_env). THE one resolution rule: both the dispatch (via spark_model’s target_defaults::resolve) and the buffer arena (via policy_from_env) call this, so the split count a launch uses and the split count the workspace was sized for cannot disagree.