refactor(lib-blockchain): Move difficulty adjustment ownership to lib-consensus

[umwelt]

Description

Part of issue #605: Adaptive Difficulty Adjustment for PoUW Consensus

Architectural improvement: Move difficulty adjustment policy ownership from lib-blockchain to lib-consensus, following the principle "consensus engine should own difficulty policy".

Current State

• Difficulty adjustment logic in lib-blockchain::blockchain.rs
• DAO proposal system in lib-consensus
• Parameter governance split between two crates

Desired State

• lib-consensus owns difficulty adjustment algorithm
• lib-consensus owns DifficultyConfig
• lib-blockchain calls lib-consensus::adjust_difficulty()
• lib-consensus manages DAO parameter updates
• Clear separation of concerns

Acceptance Criteria

• Create lib-consensus::difficulty_manager module
• Move DifficultyConfig from lib-blockchain to lib-consensus
• Create DifficultyManager struct in lib-consensus
• Implement adjust_difficulty() in DifficultyManager
• Update BlockchainConsensusCoordinator to manage difficulty updates
• Update lib-blockchain to call lib-consensus::DifficultyManager
• All existing tests pass after refactoring
• Add integration tests between crates
• Update documentation to reflect architecture

Priority

• MEDIUM - Nice to have for clean architecture
• Can be deferred until after feat: Implement adaptive difficulty adjustment for network-wide PoUW consensus #605 core implementation

Related

• Blocks issue feat: Implement adaptive difficulty adjustment for network-wide PoUW consensus #605: Adaptive Difficulty Adjustment for PoUW Consensus