Module compression 

Zstd compression and bincode serialization for block bodies and headers.

This module owns all serialization/deserialization logic:

• BlockStore::serialize_block / BlockStore::deserialize_block: bincode + zstd (with optional dictionary).
• BlockStore::serialize_header / BlockStore::deserialize_header: bincode only (no compression).
• Dictionary training: BlockStore::train_dictionary, BlockStore::maybe_train_dictionary.
• Dictionary loading: [resolve_zstd_dictionary], [load_zstd_dict_from_db].

Requirements

• SER-001 — block serialization.
• SER-002 — header serialization.
• SER-005 — dictionary training.