Module header 

L2BlockHeader — independently hashable L2 block metadata and commitments.

Requirements:

• BLK-001 — field groups / NORMATIVE § BLK-001
• BLK-002 — constructors / NORMATIVE § BLK-002
• BLK-007 — version auto-detection / NORMATIVE (BLK-007)
• SVL-001 — header version vs height / DFSP activation (L2BlockHeader::validate)
• SVL-002 — DFSP roots must be EMPTY_ROOT before activation (L2BlockHeader::validate_with_dfsp_activation)
• SVL-003 — declared L2BlockHeader::total_cost / L2BlockHeader::block_size vs protocol caps (crate::MAX_COST_PER_BLOCK, crate::MAX_BLOCK_SIZE)
• SVL-004 — L2BlockHeader::timestamp vs wall clock + crate::MAX_FUTURE_TIMESTAMP_SECONDS; production [validate]/L2BlockHeader::validate_with_dfsp_activation, tests L2BlockHeader::validate_with_dfsp_activation_at_unix
• HSH-001 — header hash() (SPEC §3.1 field order; preimage length L2BlockHeader::HASH_PREIMAGE_LEN)
• SER-002 — L2BlockHeader::to_bytes / L2BlockHeader::from_bytes (SPEC §8.2; bincode + BlockError::InvalidData on decode)
• SER-003 — L2BlockHeader::genesis deterministic bootstrap (SPEC §8.3; see NORMATIVE § SER-003)
• SPEC §2.2, SPEC §8.3 Genesis

Usage

Prefer L2BlockHeader::new, L2BlockHeader::new_with_collateral, L2BlockHeader::new_with_l1_proofs, or L2BlockHeader::genesis so version is never caller-supplied (auto-detected from height; shared rules in L2BlockHeader::protocol_version_for_height and L2BlockHeader::protocol_version_for_height_with_activation (BLK-007). Production code that needs wall-clock timestamps should set timestamp after new() or use [crate::builder::BlockBuilder] (BLD-005): L2BlockHeader::new leaves timestamp at 0 per SPEC’s derived-new() parameter list.

Field order matches SPEC §2.2 so bincode layout stays deterministic (SER-001, HSH-001). Canonical encode/decode helpers live on L2BlockHeader::to_bytes / L2BlockHeader::from_bytes (SER-002).

Rationale

Splitting header from body (super::block::L2Block, BLK-003) mirrors an Ethereum-style header/body split: attestations and light clients can process headers without deserializing SpendBundle payloads.

Decisions

• Bytes32 and Cost come from crate::primitives so this crate has one type identity for hashes and CLVM cost (BLK-006).
• L1 proof anchors are Option<Bytes32>; omitted proofs serialize as None (default) per SPEC.
• DFSP roots are mandatory Bytes32 fields; pre-activation they are set to crate::EMPTY_ROOT by constructors / validation (SVL-002), not by the type itself.

Structs

L2BlockHeader

DIG L2 block header: identity, Merkle commitments, L1 anchor, metadata, optional L1 proofs, slash proposal commitments, and DFSP data-layer roots.