feat: share serialization/deserialization

[lonerapier]

Motivation

Allow persistent and interoperable storage of Share values with versioned formats behind a feature flag and optional serde integration

Approach

• Implement a compact binary format: [magic][version][field_id][poly_id][x][y][checksum?]
• Provide APIs:
  • Share::to_bytes() -> Vec<u8>
  • Share::from_bytes(&[u8]) -> Result<Self, ShareError>
  • Add optional serde feature

Tasks

• Define and document share format.
• Implement encode/decode functions.
• Integrate serde (behind feature flag).
• Add roundtrip tests and fuzzing.