Wire enable_cuda_graph_decode through engine args, stage config and CLI

Follows the vae_use_tiling path: OmniDiffusionConfig field, OmniEngineArgs,
StageDeployConfig, _DiffusionConfigProjection and --[no-]enable-cuda-graph-decode.
Documents interplay with enforce_eager and cudagraph_mode.

vllm_omni/engine/arg_utils.py

@@ -574,6 +574,10 @@ class OrchestratorArgs:
     diffusion_attention_config: str | None = None
     diffusion_compile_granularity: str | None = None
     diffusion_compile_dynamic: bool | None = None
+    # CUDA graph capture of fixed-shape KV-cache decode steps (Qwen-Image-2.1
+    # today). None defers to the OmniDiffusionConfig default (enabled);
+    # --enforce-eager also disables it.
+    enable_cuda_graph_decode: bool | None = None
     cache_backend: str = "none"
     cache_config: str | None = None
     video_output_transport: dict[str, object] | None = None