Ruby port of mlx-examples/stable_diffusion with a lightweight synthetic-first pipeline.

• stable_diffusion/config.rb: model config objects.
• stable_diffusion/tokenizer.rb: tokenizer utility.
• stable_diffusion/clip.rb: CLIP-like text encoder.
• stable_diffusion/unet.rb: diffusion denoiser model.
• stable_diffusion/vae.rb: latent autoencoder.
• stable_diffusion/sampler.rb: Euler and Euler-ancestral samplers.
• stable_diffusion/model_io.rb: model loading builders.
• stable_diffusion/__init__.rb: StableDiffusion and StableDiffusionXL pipelines.
• txt2image.rb: prompt-to-image CLI (writes PPM output).
• image2image.rb: image-to-image CLI (reads/writes PPM).
• test.rb: synthetic pipeline tests.

Text-to-image:

ruby stable_diffusion/txt2image.rb --model sdxl --n-images 2 --steps 2 "A red cube on a table"

Image-to-image:

ruby stable_diffusion/image2image.rb input.ppm "A lit fireplace"

ruby stable_diffusion/test.rb