Function pair_request_kernel 

pub fn pair_request_kernel(
    left: [b8; 32],
    right: [b8; 32],
    tag: b32,
    valid: bool,
) -> CompressionInput

A single-block BLAKE3 root hash of left ‖ right.

Builds the lane request for one Merkle-top node: IV key, 64-byte block, counter 0, single-block root flags. tag (the heap index) comes back with the result; valid says whether the request is real. Combinational.