chore: OpenVINO MetricGAN GPU spike (do not merge)

Arc B580 lost the kill test: batch-14 mask p50 103 ms vs Torch CPU
70 ms enhance / OpenVINO CPU 5 ms. Production stays on known-good
CPU workers (tag known-good-cpu-2026-08-28). Hop stays 200 ms.

scripts/spike_export_metricgan_onnx.py

@@ -0,0 +1,58 @@
+#!/usr/bin/env python3
+"""Export MetricGAN enhance_model (BLSTM masker) to ONNX. Uses project .venv."""
+from __future__ import annotations
+
+import argparse
+from pathlib import Path
+
+import torch
+import torch.nn as nn
+from speechbrain.inference.enhancement import SpectralMaskEnhancement
+
+SAVEDIR = "/root/.cache/speechbrain-enhancement"
+
+
+class MaskNet(nn.Module):
+    def __init__(self, net: nn.Module):
+        super().__init__()
+        self.net = net
+
+    def forward(self, feats: torch.Tensor) -> torch.Tensor:
+        # lengths=1.0 relative, one per batch row
+        lengths = torch.ones(feats.shape[0], dtype=feats.dtype)
+        return self.net(feats, lengths=lengths)
+
+
+def main() -> None:
+    ap = argparse.ArgumentParser()
+    ap.add_argument("--out", default="scripts/metricgan_enhance.onnx")
+    args = ap.parse_args()
+    out = Path(args.out)
+    out.parent.mkdir(parents=True, exist_ok=True)
+
+    m = SpectralMaskEnhancement.from_hparams(
+        source="speechbrain/metricgan-plus-voicebank",
+        savedir=SAVEDIR,
+        run_opts={"device": "cpu"},
+    )
+    wrap = MaskNet(m.mods.enhance_model).eval()
+    feats = m.compute_features(torch.randn(1, 16000))
+    with torch.no_grad():
+        torch.onnx.export(
+            wrap,
+            feats,
+            str(out),
+            input_names=["feats"],
+            output_names=["mask"],
+            dynamic_axes={
+                "feats": {0: "batch", 1: "time"},
+                "mask": {0: "batch", 1: "time"},
+            },
+            opset_version=17,
+            dynamo=False,
+        )
+    print("wrote", out, "bytes", out.stat().st_size, "example_feats", tuple(feats.shape))
+
+
+if __name__ == "__main__":
+    main()