Module checkpoint_builder 

Incremental construction of crate::Checkpoint for an L2 epoch (CKP-006).

Requirements trace

• CKP-006 — CheckpointBuilder: new, add_block, set_state_root, add_withdrawal, build.
• NORMATIVE § CKP-006 — builder obligations for epoch summaries.
• SPEC §6.6 — checkpoint builder in block production.
• Merkle roots: CheckpointBuilder::build uses the crate-internal merkle_tree_root helper — same tagged binary Merkle tree as crate::L2Block::compute_spends_root / crate::L2Block::compute_slash_proposals_root (BLK-004, HSH-007 via chia_sdk_types::MerkleTree). Empty leaf lists → crate::EMPTY_ROOT (CKP-006 implementation notes).
• HSH-002 — crate::Checkpoint::hash applies once the built Checkpoint is complete.

Rationale

• Consuming build: CheckpointBuilder::build takes self by value so a finalized crate::Checkpoint cannot be accidentally extended (CKP-006 implementation notes).
• Ordered leaves: Block hashes and withdrawal hashes are Merkle leaves in append order — matches CheckpointBuilder::add_block / CheckpointBuilder::add_withdrawal call order during the epoch.
• State root: Defaults to Bytes32::default until CheckpointBuilder::set_state_root runs; production code should set the post-epoch trie root before signing.

Structs

CheckpointBuilder

Accumulates per-epoch block hashes, fees, tx counts, and withdrawal commitments.