New contract outputs were derived with next_child(), which uses the wallet's
active account. During a late-lock the active account can differ from the one the
contract context was created under, so an output could be created in the wrong
account. Derive under context.parent_key_id instead.
- WalletBackend: add next_child_for(parent_key_id, mask); next_child delegates to it.
- add_outputs_to_ctx derives via next_child_for(&context.parent_key_id, ..).
- parent_key_for returns Result and errors (UnknownAccountLabel) on an unknown
account name instead of silently falling back to the active account.
- test: a contract referencing an unknown account now errors.
Addresses PR #754 review (context.rs, utils.rs).