[SC-11] Implement OwnershipRecord struct and ownership management

[yusuftomilola]

Overview

Define the OwnershipRecord struct and implement functions for managing fractional token ownership inside contracts/contrib/.

Location

All work must be inside contracts/contrib/src/tokenization.rs.

Acceptance Criteria

• OwnershipRecord struct: owner: Address, balance: i128, locked_balance: i128, acquisition_timestamp: u64, voting_power: i128, unclaimed_dividends: i128
• get_ownership_record(env, asset_id, owner) -> OwnershipRecord — returns ownership record
• get_all_holders(env, asset_id) -> Vec<Address> — returns all addresses with a balance > 0
• ownership_percentage(env, asset_id, owner) -> i128 — returns percentage in basis points (10000 = 100%)