feat: Add fs store to block manager

[DerekCuevas]

Note

Introduces filesystem-backed storage for blocks and enables JSON (de)serialization across core types.

• Implements BlockManager persistence: new data_dir, persist_block, load_from_disk, add_block_internal(persist), and constructors new/new_with_load; blocks saved as <hash>.json in ./data and loaded with orphan handling
• Adds Serialize/Deserialize to Hash, BlockHeader, Block, Transaction*, and Address; custom serde for Signature and PublicKey (hex encoding); wraps raw [u8; 32] in Hash type and updates callers
• Updates chain to consume Hash via as_ref(); minor logging/hex fixes using hash.0
• Adds tests for persistence and loading; updates .gitignore to ignore ./data; adds serde_json dependency

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[cursor]

Removed blocks reappear after loading from disk

The remove_block function only removes blocks from memory but doesn't delete the corresponding JSON file from disk. Since add_block persists blocks via persist_block, this creates an inconsistency where removed blocks will reappear after calling load_from_disk(). Any block that was added, removed, and then loaded from disk will be restored to memory unexpectedly.

Additional Locations (1)

• src/block_manager.rs#L171-L184