[Model] Add BAGEL step execution and continuous batching (#6359)

docs/design/feature/diffusion_continuous_batching.md

@@ -150,8 +150,17 @@ that do not belong in the shared contract should be stored in `state.extra`.
 Queueing and lifecycle metadata remain in the scheduler's request state.
 
 Current native pipelines that explicitly enable step execution include
-Qwen-Image, HunyuanImage3, and Helios. Step execution alone does not imply
-continuous-batching support: Qwen-Image accepts batched step states.
+Qwen-Image, BAGEL, HunyuanImage3, and Helios. Step execution alone does not
+imply continuous-batching support: Qwen-Image accepts batched step states.
+BAGEL accepts batched step states for image generation with its default,
+think-mode, and single-stage deploy configs. Only the diffusion stage uses the
+step protocol in two-stage deployments; explicit single-stage text output
+retains complete-request execution. BAGEL schedules exactly
+`num_inference_steps` denoising updates, including a one-step request. When a
+finished stepwise request is followed by an explicit text fallback wave, the
+runner retires its step state and paged diffusion-KV rows before the full
+forward dispatch. BAGEL's step path does not currently support sequence
+parallelism or a diffusion cache backend.
 HunyuanImage3 accepts batched step states only when its resolved self-attention
 backend is `TORCH_SDPA`; otherwise it rejects groups larger than one request.
 Configure `DIFFUSION_ATTENTION_BACKEND=TORCH_SDPA` or