[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>

tests/config/test_import_guard.py

@@ -0,0 +1,36 @@
+"""Import guard for AR/TTS-only deployments: heavy diffusion dependencies stay unloaded (CPU).
+
+Runs in a subprocess so the assertion is not polluted by modules other tests
+imported into this interpreter.
+"""
+
+from __future__ import annotations
+
+import json
+import subprocess
+import sys
+
+import pytest
+
+pytestmark = [pytest.mark.core_model, pytest.mark.cpu]
+
+_PROBE = r"""
+import json, sys, time
+t = time.perf_counter(); import vllm; t_vllm = time.perf_counter() - t
+t = time.perf_counter(); import vllm_omni; t_omni = time.perf_counter() - t
+from vllm_omni.config.config_factory import StageConfigFactory  # noqa: F401
+from vllm_omni.entrypoints.async_omni import AsyncOmni  # noqa: F401
+from vllm_omni.config.pipeline_registry import OMNI_PIPELINES
+assert "qwen3_tts" in OMNI_PIPELINES
+heavy = ["diffusers", "x_transformers", "whisper", "vllm_omni.diffusion.registry", "vllm_omni.diffusion.io_support"]
+print(json.dumps({"t_vllm": t_vllm, "t_omni": t_omni, "loaded": [m for m in heavy if m in sys.modules]}))
+"""
+
+
+def test_tts_path_does_not_import_diffusion_stack():
+    out = subprocess.run([sys.executable, "-c", _PROBE], capture_output=True, text=True, timeout=600)
+    assert out.returncode == 0, out.stderr[-2000:]
+    info = json.loads(out.stdout.strip().splitlines()[-1])
+    assert info["loaded"] == [], f"heavy modules imported on the AR/TTS path: {info['loaded']}"
+    # Import overhead of vllm_omni over bare vllm is reported (not gated: it is host dependent).
+    print(f"import vllm {info['t_vllm']:.1f}s, +vllm_omni {info['t_omni']:.1f}s")