Module checkpoint 

Checkpoint domain types: Checkpoint (CKP-001), CheckpointSubmission (CKP-002).

Requirements trace

• CKP-001 — Checkpoint: nine public fields + Checkpoint::new default instance.
• CKP-002 — CheckpointSubmission: checkpoint + crate::SignerBitmap + aggregate BLS + score + submitter + L1 tracking options.
• NORMATIVE § CKP-001 / CKP-002 — checkpoint + submission field layouts and constructors.
• SPEC §2.6 — checkpoint as epoch summary anchored toward L1.
• CKP-004 — Checkpoint::compute_score: stake_percentage * block_count (epoch competition score).
• CKP-005 — CheckpointSubmission: CheckpointSubmission::hash, CheckpointSubmission::epoch, threshold helpers, L1 CheckpointSubmission::record_submission.
• CKP-006 — crate::CheckpointBuilder accumulates block / withdrawal hashes and produces block_root / withdrawals_root using the same internal Merkle helper as BLK-004 (merkle_tree_root in merkle_util.rs).
• HSH-002 / SPEC §3.2 — Checkpoint::hash: SHA-256 over 160-byte fixed-order preimage (chia_sha2::Sha256).
• SER-001 — bincode via Serialize / Deserialize on wire-bearing structs.
• SER-002 — Checkpoint::to_bytes / Checkpoint::from_bytes and CheckpointSubmission::to_bytes / CheckpointSubmission::from_bytes with CheckpointError::InvalidData on decode failures.

Rationale

• Public fields: Same ergonomics as crate::types::receipt::Receipt — consensus / builder layers assign values; this crate stays a typed bag of record (CKP-001 acceptance: read/write access).
• Default roots: Bytes32::default is the all-zero hash, matching “empty Merkle” conventions used elsewhere (crate::constants::EMPTY_ROOT is the documented empty-tree sentinel; callers may normalize roots when building real checkpoints — CKP-006).
• CheckpointSubmission + L1 options: submission_height / submission_coin start None at construction; CheckpointSubmission::record_submission persists L1 proof (CKP-005, CKP-002 notes).

Structs

Checkpoint

Epoch summary checkpoint: aggregate stats and Merkle roots for one L1-anchored epoch (SPEC §2.6, CKP-001).

CheckpointSubmission

Signed checkpoint submission: epoch summary plus validator attestation material (SPEC §2.7, CKP-002).