Function certificate_request 

pub fn certificate_request(
    epoch: b64,
    batch_id: b64,
    root: [b8; 32],
    tag: b16,
) -> SignRequest

Pack the actual DSig fixed-64 message into the actual Ed25519 request type.

Calls batch_commitment_words directly. No host prehash, duplicated domain/profile constants, endian conversion, or intermediate register is inserted. The returned tag is routing metadata, not a signed field.

Inputs: 64-bit epoch and batch_id, the 32-byte root and the 16-bit tag. Output: the core’s SignRequest (eight 64-bit message words plus the tag). Timing: combinational, zero cycles.