test(smoke): add gpu smoke for minimum generate path

tests/test_smoke_gpu.py

@@ -0,0 +1,51 @@
+"""GPU smoke tests — skipped by default. Opt in with: pytest -m gpu
+
+Generates the minimum-viable songs end-to-end through the real ACE-Step
+pipeline. Run before each release tag.
+
+Skipped automatically in CI by the pyproject ``addopts = -m 'not gpu'``
+default. Requires:
+
+- ``ace-step`` installed (Apple Silicon fork on Mac, upstream on CUDA)
+- First run downloads ACE-Step 1.5 XL SFT weights (~16 GB) into the HF cache
+- A real MPS / CUDA device — CPU inference is functionally untested
+"""
+
+from __future__ import annotations
+
+import os
+from pathlib import Path
+
+import pytest
+
+pytestmark = pytest.mark.gpu
+
+
+def test_generate_minimum_song(tmp_path):
+    """Smallest end-to-end: 5 s instrumental drone, seed=1."""
+    os.environ.setdefault("ACE_MODEL_PATH", "ACE-Step/ACE-Step-v1.5-XL-SFT")
+
+    from backend import ACEStepStudioBackend
+
+    b = ACEStepStudioBackend()
+    out_path, meta = b.dispatch(
+        mode="generate",
+        params={
+            "prompt": "test tone, simple drone",
+            "lyrics": "[intro] tone",
+            "duration_s": 5,
+            "instrumental": True,
+            "seed": 1,
+            "loras": [],
+            "advanced": {},
+            "lm": {},
+            "dcw": {},
+        },
+    )
+    assert Path(out_path).exists()
+    assert Path(out_path).stat().st_size > 0
+    assert meta["mode"] == "generate"
+    assert meta["seed"] == 1
+    # Wall time should be < 5 min even on first cold run + 16 GB weight download.
+    # Subsequent runs should be < 30 s on M5 Max.
+    assert meta["wall_seconds"] > 0