[Edge][WP6] Slim the AR/TTS import path and add an edge dependency set

* diffusers is no longer imported by vllm_omni.diffusion.data (annotation only);
  vllm_omni.diffusion.io_support (diffusion registry + distributed state) is
  imported lazily by config_factory and async_omni_engine, so constructing a
  Qwen3-TTS pipeline config / AsyncOmni no longer loads diffusers,
  x_transformers or whisper (guarded by tests/config/test_import_guard.py).
* requirements/common.txt = core.txt (AR/speech runtime) + media.txt
  (diffusion/video/extra audio); VLLM_OMNI_EDGE_BUILD=1 makes setup.py use
  requirements/<device>-edge.txt (core only); `edge` extra adds websockets and
  sounddevice for on-device clients.

Measured on this host (CUDA venv): `import vllm` 7.0-7.9 s, `import vllm_omni`
adds ~20 s of which ~17 s is transformers' own import (transformers.utils.import_utils)
pulled in by vllm_omni.patch / vllm_omni.version; that floor is not
vLLM-Omni's and is unchanged. Guard test: no diffusion module loaded on the TTS path.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

vllm_omni/diffusion/data.py

@@ -10,7 +10,9 @@ from dataclasses import dataclass, field, fields
 from enum import Enum
 from typing import TYPE_CHECKING, Any
 
-import diffusers
+if TYPE_CHECKING:  # heavy; only needed for annotations here
+    import diffusers
+
 import huggingface_hub
 import torch
 from PIL import Image
@@ -897,7 +899,7 @@ class OmniDiffusionConfig:
     # kwargs forwarded to pipeline.__call__()
     diffusers_call_kwargs: dict[str, Any] = field(default_factory=dict)
     # Actual diffusers pipeline object (to determine inputs of the dummy run)
-    diffusers_pipeline_cls: type[diffusers.DiffusionPipeline] | None = None  # pyright: ignore[reportPrivateImportUsage]
+    diffusers_pipeline_cls: "type[diffusers.DiffusionPipeline] | None" = None  # pyright: ignore[reportPrivateImportUsage]
 
     # http server endpoint config, would be ignored in local mode
     host: str | None = None