feat: known-good CPU audio + 1080p VAAPI + KMS display wall

Working snapshot before the OpenVINO MetricGAN GPU spike:
thread-capped SpeechBrain DelaySum+MetricGAN on CPU, hop 200 ms,
video hold for A/V sync, Arc VAAPI H.264, headless kmssink wall.
32 tests passing. Do not treat this as a GPU audio path.

tests/test_drm_outputs.py

@@ -0,0 +1,21 @@
+"""DRM connector discovery for the 3-display LiveKit wall."""
+from drm_outputs import list_outputs, select_outputs
+
+
+def test_lists_arc_hdmi():
+    outs = list_outputs()
+    assert outs, "expected at least one DRM connector"
+    names = {o.name for o in outs}
+    assert "HDMI-A-7" in names
+    hdmi7 = next(o for o in outs if o.name == "HDMI-A-7")
+    assert hdmi7.connector_id > 0
+    assert hdmi7.driver in ("xe", "i915")
+    assert hdmi7.node.startswith("/dev/dri/card")
+
+
+def test_select_three_prefers_xe_hdmi():
+    three = select_outputs(n=3)
+    assert len(three) == 3
+    assert three[0].name == "HDMI-A-7" or three[0].connected
+    ids = [o.connector_id for o in three]
+    assert len(set(ids)) == 3