[SC-13] Implement lock_tokens and unlock_tokens functions

[yusuftomilola]

Overview

Implement lock_tokens and unlock_tokens functions to restrict token transferability inside contracts/contrib/.

Location

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

Acceptance Criteria

• lock_tokens(env, asset_id, owner, amount) — admin or token owner only; moves amount from transferable to locked_balance in OwnershipRecord; panics if insufficient transferable balance
• unlock_tokens(env, asset_id, owner, amount) — admin or token owner only; moves amount from locked_balance back to transferable; panics if insufficient locked balance
• get_locked_balance(env, asset_id, owner) -> i128 — returns locked amount
• Emits lock/unlock events with asset_id, owner, amount