fix: run VAE in fp32 to stop black/blank generated images

fp16 VAE produces NaNs on many SD1.5/SDXL checkpoints, yielding black
images (often mistaken for NSFW censorship - there is no NSFW filter in
the generator). Switch COMFYUI_FLAGS from --fp16-vae to --fp32-vae in
.env.example and document the fix in the README. Verified end-to-end:
a test generation now produces a real image (mean ~86) instead of black.

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

README.md

@@ -40,10 +40,15 @@ The DGX Spark's Grace-Blackwell architecture uses **unified memory** - a coheren
 ```bash
 --disable-pinned-memory   # Reduces overhead on unified fabric
 --force-fp16              # Enables SageAttention optimization
---fp16-unet --fp16-vae --fp16-text-enc  # FP16 precision throughout
+--fp16-unet --fp16-text-enc  # FP16 precision for UNet + text encoder
+--fp32-vae               # VAE in fp32 - fp16 VAE causes NaNs -> BLACK images
 --dont-upcast-attention   # Keeps attention in FP16 for speed
 ```
 
+> **Black/blank images?** That's the classic fp16-VAE NaN issue, not an NSFW
+> filter (there is none). Keep `--fp32-vae` (default). `--bf16-vae` is a faster
+> alternative that also avoids the NaNs.
+
 **What NOT to use:**
 - `--gpu-only` - fights the unified memory fabric, hurts performance
 - `--cache-none` - disables natural caching, slows model loading