Constant GDN_TC_SPINE_ENTRY 

pub const GDN_TC_SPINE_ENTRY: &str = "gated_delta_rule_chunk_delta_h_tcfuse_x2";

The spine entry the [defaults] gdn_prefill_tc lever ships.

_x2 = two bf16 limbs of S_c in Phase A. Both entries are ABI-, grid-, block- and smem-identical, so the choice is invisible downstream and only the accuracy contract separates them (see the module docs).