Function gdn_tc_spine_route_line 

pub fn gdn_tc_spine_route_line(
    num_v_heads: u32,
    batch_size: u32,
    smem_bytes: u32,
) -> String

2026-09-25: GDN state spine: …, the line the prefill logs when the tensor-core spine runs, built from GDN_TC_SPINE_ENTRY. Pure and returning a String, so the tests below can check it.