feat(diffusion): add request-gated VAE fast paths

Signed-off-by: Hongsheng Liu <liuhongsheng4@huawei.com>

vllm_omni/entrypoints/cli/serve.py

@@ -629,6 +629,15 @@ class OmniServeCommand(CLISubcommand):
             action="store_true",
             help="Enable VAE tiling for memory optimization (useful for mitigating OOM issues).",
         )
+        omni_config_group.add_argument(
+            "--vae-decode-precision",
+            choices=("fp32", "fp16", "bf16"),
+            default=None,
+            help=(
+                "Optional Wan decode-only VAE precision. The encoder remains FP32; "
+                "unset preserves the pipeline's existing precision behavior."
+            ),
+        )
 
         # Parallel weight loading (faster diffusion startup)
         omni_config_group.add_argument(