[SC-06] Implement owner registry (address to asset IDs index)

[yusuftomilola]

Overview

Implement the owner registry that maps each address to a list of asset IDs it owns inside contracts/contrib/.

Location

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

Acceptance Criteria

• get_assets_by_owner(env, owner) -> Vec<BytesN<32>> — returns all asset IDs for the given address
• add_to_owner_registry(env, owner, asset_id) — appends asset ID to owner list (internal helper)
• remove_from_owner_registry(env, owner, asset_id) — removes asset ID from owner list (internal helper)
• Both helpers called automatically in register_asset and transfer_asset
• Registry stored under DataKey::OwnerAssets(Address)