feat(device): support xpu backend (#24)

* feat(device): support xpu backend

* Fall back to CPU seed generator when device RNG unsupported (xpu)

Some torch-xpu builds have no device-side RNG, so torch.Generator(device="xpu")
raises when --seed is used. _make_seed_generator tries the device generator and
falls back to a backend-agnostic CPU generator. Adds a fallback unit test.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Victor Kuznetsov <kuznetsov.va@gmail.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/remove_ai_watermarks/invisible_engine.py

@@ -88,7 +88,7 @@ class InvisibleEngine:
 
         Args:
             model_id: HuggingFace model ID. None = use default for pipeline.
-            device: Device for inference (auto/cpu/mps/cuda). None = auto.
+            device: Device for inference (auto/cpu/mps/cuda/xpu). None = auto.
             pipeline: Pipeline profile. "default" (SDXL base, defeats SynthID
                 v2) or "ctrlregen" (CtrlRegen).
             hf_token: HuggingFace API token.