Constant GATEUP_FUSED_MAX_M 

pub const GATEUP_FUSED_MAX_M: usize = 16;

2026-09-25: The widest M the fused dense-FFN gate+up decode GEMM serves, and so the row extent of ffn_gate_up_fused. Declared here because the arena is sized in this crate; the dispatch (layers/dense_ffn_gateup_fused.rs in metrale-model-layers) reads it for its 5..=GATEUP_FUSED_MAX_M band, so the band and the buffer cannot disagree.