[Fix] Make lua_shape :transform() tableshape-compatible

[Fix] Make lua_shape :transform() tableshape-compatible

Change :transform() return values to match tableshape behavior:
- Success: return value (not true, value)
- Failure: return nil, error (not false, error)

This is required for compatibility with lua_selectors check_args which
expects the tableshape return convention. The :check() method still
returns (bool, value_or_error) for new code.