[SC-10] Implement TokenizedAsset struct and tokenize_asset function

[yusuftomilola]

Overview

Define the tokenization structs and implement the tokenize_asset function that converts a registered asset into fractional tokens inside contracts/contrib/.

Location

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

Acceptance Criteria

• TokenizedAsset struct with fields: asset_id, total_supply: i128, symbol: String, decimals: u32, tokenizer: Address, valuation: i128, revenue_sharing_enabled: bool, tokenization_timestamp: u64
• TokenMetadata struct: name, description, asset_type (Physical/Digital), ipfs_uri, legal_docs_hash, accredited_investor_required
• tokenize_asset(env, asset_id, symbol, total_supply, valuation, metadata) — owner only; stores TokenizedAsset; creates initial OwnershipRecord for tokenizer with 100% ownership; emits event
• Panics if asset not found, already tokenized, or contract paused