pub const ZERO_HASH: Bytes32;Expand description
32 zero bytes — a distinct sentinel from EMPTY_ROOT.
Rationale: Header hashing and optional fields use an all-zero hash to mean “absent” or “zeroed
placeholder” per SPEC; Merkle empty roots must still use EMPTY_ROOT. Keeping both constants avoids
ambiguous “empty” semantics that would break second-layer verifiers.