[Contract] Implement Asset Metadata Update, Ownership Transfer, and Retirement

[yusuftomilola]

Post-registration lifecycle operations: updating an asset's mutable metadata, transferring ownership between addresses, and marking an asset as retired.

Requirements

update_asset_metadata(env, asset_id, new_description, new_metadata_uri, new_custom_attributes, caller):

• Caller must be asset owner or admin
• Applies provided Option<T> values (skips None)
• Validates metadata URI if provided
• Emits asset_upd event

transfer_asset_ownership(env, asset_id, new_owner, caller):

• Caller must be current owner
• New owner cannot be zero address
• Removes ID from old owner's registry, adds to new owner's registry
• Updates asset.owner, last_transfer_timestamp, status = Transferred
• Emits asset_tx event

retire_asset(env, asset_id, caller):

• Caller must be owner or admin
• Sets status = Retired
• Emits asset_ret event

Acceptance Criteria

• Metadata update with all None args is a no-op (no error)
• Ownership transfer correctly updates both registries
• Retired assets cannot be re-transferred (check status in transfer)