InvisibleEngine substituted DEFAULT_MODEL_ID whenever model_id was None. When
b0ca205 tightened the remover's fixed-stack check from `not in {None,
DEFAULT_MODEL_ID}` to `is not None`, that substitution turned every single
InvisibleEngine construction into a ValueError - including the deployed Modal
worker's setup(), which is how it was found.
The library suite missed it because these two are tested from opposite sides:
every remover test builds WatermarkRemover directly with model_id unset, and
every engine test mocks the remover away. Nothing exercised the seam between
them. TestEngineDoesNotFabricateAModelId now does, without a GPU.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
