feat(lib-blockchain): Add DifficultyConfig struct to blockchain state

[umwelt]

Description

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

Add a new DifficultyConfig struct to store governance-controlled difficulty parameters in the blockchain state, replacing hardcoded constants.

Acceptance Criteria

• Create DifficultyConfig struct in lib-blockchain with fields:
  • target_timespan: u64 (default: 14 days)
  • adjustment_interval: u64 (default: 2016 blocks)
  • min_adjustment_factor: u64 (default: 4 - max 4x decrease)
  • max_adjustment_factor: u64 (default: 4 - max 4x increase)
  • last_updated_at_height: u64 (tracks when last changed)
• Add difficulty_config: DifficultyConfig field to Blockchain struct
• Initialize with default values in Blockchain::new()
• Implement serialization/deserialization for persistence
• Add getter method get_difficulty_config(&self) -> &DifficultyConfig
• Write unit tests for serialization round-trip
• Ensure backward compatibility with existing genesis blocks

Related

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