feat(invisible): add Qwen-Image img2img pipeline (--pipeline qwen)

A third diffusion pipeline alongside sdxl/controlnet: Qwen-Image (20B MMDiT,
Apache-2.0 code AND weights) img2img. The scrub still comes from the img2img
strength; Qwen preserves text (incl. CJK) and structure markedly better than
SDXL at the scrub floor, so it over-regenerates real photos far less (directly
targets the controlnet over-regeneration that degrades real uploads).

- watermark_profiles: QWEN_MODEL_ID, normalize_profile accepts "qwen".
- WatermarkRemover: _load_qwen_pipeline (bf16, loads Qwen base unless --model
  overridden, clear ImportError if diffusers lacks the class), _run_qwen (no
  MPS fallback -- 20B is CUDA/cloud-class), dispatch in _generate_one/preload,
  pure _build_qwen_kwargs (true_cfg_scale, not guidance_scale).
- Shared _base_load_kwargs() across all three loaders (dtype + token).
- CLI --pipeline gains "qwen"; invisible_engine threads it through.
- scripts/qwen_scrub_prototype.py: standalone PEP 723 GPU experiment.

Prototype oracle floors (Modal A100-80GB, single seed, controls SynthID-positive,
PENDING seed-repeat cert): OpenAI clears at strength ~0.10, Gemini at ~0.30 (0.20
still detected), with CJK text + faces faithful where controlnet plasticizes. The
Gemini floor is higher than the shared default ladder, so pass an explicit
--strength for Gemini on this pipeline until a Qwen-specific ladder is certified.

The model-running path is CUDA-only (untestable locally); unit tests cover the
pure call-shape (_build_qwen_kwargs) and profile normalization without torch.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/remove_ai_watermarks/cli.py

@@ -253,15 +253,16 @@ def _normalize_pipeline(ctx: click.Context, param: click.Parameter, value: str |
     return normalized
 
 
-# ``controlnet`` (the default-SELECTED value) and ``sdxl`` (plain SDXL img2img) are the
-# two current profiles; ``default`` is an OUTDATED back-compat alias for ``sdxl``
-# (warned + normalized away by _normalize_pipeline).
-_PIPELINE_CHOICES = ["sdxl", "controlnet", "default"]
+# ``controlnet`` (the default-SELECTED value), ``sdxl`` (plain SDXL img2img) and
+# ``qwen`` (Qwen-Image, CUDA/cloud-class) are the current profiles; ``default`` is an
+# OUTDATED back-compat alias for ``sdxl`` (warned + normalized away by _normalize_pipeline).
+_PIPELINE_CHOICES = ["sdxl", "controlnet", "qwen", "default"]
 _PIPELINE_HELP = (
     "Pipeline profile. controlnet (DEFAULT) = SDXL + canny ControlNet that preserves "
     "text/faces via edge conditioning while removing SynthID; sdxl = plain SDXL img2img "
-    "(lighter, no extra model download, but leaves SynthID on flat-graphic content). "
-    "('default' is an OUTDATED alias for 'sdxl' -- use sdxl or controlnet.)"
+    "(lighter, no extra model download, but leaves SynthID on flat-graphic content); "
+    "qwen = Qwen-Image (20B, Apache-2.0) img2img, best text/structure preservation but "
+    "CUDA/cloud-class (does not fit MPS). ('default' is an OUTDATED alias for 'sdxl'.)"
 )
 
 # Shared --pipeline / --strength decorators so the three diffusion commands