Function legacy_endpoint_block_state_kernel 

pub fn legacy_endpoint_block_state_kernel(
    input: LegacyEndpointBlockInput,
) -> KeccakState

XOR one fixed 136-byte endpoint block into the preceding sponge state.

Blocks 0 through 14 contain 17 complete endpoint lanes. Block 15 contains the final 13 lanes (104 bytes), then legacy domain byte 0x01, zero fill, and final rate bit 0x80. Together these blocks cover the exact 2,144-byte endpoint string without a host-side byte buffer.