v0.8.3 — Stable Cascade (Würstchen)

Stable Cascade (Würstchen) native MLX image pipeline (#370)

Native MLX port of Stable Cascade — a 3-stage Würstchen pipeline
(prior → decoder → VQGAN), from scratch under fusion_mlx/image/cascade/.
Does not wrap mflux.

Highlights

• Unified StableCascadeUNet serves both the prior
  (switch_level=(False,) → UpDownBlock2d 1×1 mapping, no spatial
  change) and the decoder (switch_level=None → Conv2d(k=2,s=2) down
  / ConvTranspose2d(k=2,s=2) up).
• PaellaVQModel decode-only VQGAN (scale_factor=0.3764).
• CLIP-ViT-bigG text encoder (dims=1280, 32 layers, 20 heads).
• DDPMWuerstchenScheduler (cosine _alpha_cumprod).
• NHWC throughout (MLX conv2d-native).

Weight-layout fix (key gotcha)

• Conv2d/DepthwiseConv2d (out,in,k,k) → OHWI (0,2,3,1)
• ConvTranspose2d (in,out,k,k) → OHWI (1,2,3,0) — different transpose
• nn.Linear / nn.Embedding left as-is (MLX stores (out,in) like
  PyTorch; transposing 2D weights on load is a bug)

Validation

• Key alignment vs real stabilityai weights: prior 1550/1550,
  decoder 1726/1726, VQGAN 121/122, CLIP 517/517.
• 24 unit tests green; real-weights E2E (512×512 / 768×768 PNG, CFG);
  server-path E2E via ImageGenEngine.
• ruff + black clean across the repo.

Wiring

• image_gen engine stable_cascade variant (auto-detected from
  cascade/wuerstchen model names) → /v1/images/generate.
• model_discovery maps StableCascadePriorPipeline /
  StableCascadeDecoderPipeline → text-to-image.
• docs: docs/cascade-image.md

Closes #370.

PyPI publish is blocked by trusted-publisher re-registration
(issue #384); Homebrew tap updated.