Fix #29 black output: use fp16-fixed SDXL VAE on fp16 GPUs

The stock SDXL VAE overflows to NaN in fp16, so the plain img2img path decodes
to an all-black image on a CUDA/XPU fp16 backend. This is the raiw.cc black
result HitaoLin reported (a 1086x1448 input came back uniformly black). cpu/mps
run fp32 and never hit it, and the differential / region-hires pipeline already
upcasts the VAE itself, so only the plain path on a fp16 GPU was exposed.

`_load_pipeline` now loads `madebyollin/sdxl-vae-fp16-fix` for the default SDXL
checkpoint when running fp16, gated by the pure helper `_needs_fp16_vae_fix`. A
custom non-SDXL model keeps its own VAE.

The decision logic is unit-tested without a download (TestFp16VaeFix). The
black->clean recovery itself needs a CUDA GPU and was not verifiable on this MPS
machine; it must be confirmed on the backend.

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

src/remove_ai_watermarks/noai/watermark_remover.py

@@ -61,6 +61,24 @@ def is_watermark_removal_available() -> bool:
     return _HAS_TORCH and _HAS_DIFFUSERS
 
 
+# Drop-in fp16-safe replacement for the SDXL VAE. The stock SDXL VAE overflows
+# to NaN in fp16 and decodes to an all-black image (issue #29: the raiw.cc black
+# result on a CUDA fp16 backend). This community VAE is numerically rescaled to
+# stay in fp16 range. SDXL-architecture only.
+_SDXL_FP16_VAE_ID = "madebyollin/sdxl-vae-fp16-fix"
+
+
+def _needs_fp16_vae_fix(model_id: str, default_model_id: str, is_fp16: bool) -> bool:
+    """Whether the plain img2img pipeline must swap in the fp16-fixed SDXL VAE.
+
+    Gated to the default SDXL checkpoint running in fp16: cpu/mps run fp32 (the
+    stock VAE is fine there) and the differential pipeline upcasts the VAE on its
+    own, so only this path on a fp16 GPU (CUDA/XPU) hits the NaN/black decode.
+    A custom non-SDXL ``model_id`` keeps its own VAE (the fix is SDXL-specific).
+    """
+    return is_fp16 and model_id == default_model_id
+
+
 _CUDA_FIX_ENV_KEY = "NOAI_CUDA_FIXED"
 
 
@@ -370,6 +388,14 @@ class WatermarkRemover:
             if self.hf_token:
                 load_kwargs["token"] = self.hf_token
 
+            # Avoid the SDXL fp16 NaN/all-black decode (issue #29) by loading the
+            # fp16-fixed VAE for the default SDXL checkpoint on a fp16 GPU.
+            if _needs_fp16_vae_fix(self.model_id, self.DEFAULT_MODEL_ID, self.torch_dtype == torch.float16):
+                from diffusers import AutoencoderKL
+
+                self._set_progress("Loading fp16-fixed SDXL VAE (avoids black output)...")
+                load_kwargs["vae"] = AutoencoderKL.from_pretrained(_SDXL_FP16_VAE_ID, torch_dtype=torch.float16)
+
             self._pipeline = AutoImg2ImgPipeline.from_pretrained(  # type: ignore
                 self.model_id,
                 **load_kwargs,