Add single-file htdemucs_fwd ONNX with embedded weights
Repacked from the existing 2-file export (htdemucs_fwd.onnx +
.onnx.data) so consumers that expect one self-contained file work
without needing external-data support. Weights embedded via
onnx.save(..., save_as_external_data=False); verified bit-identical
(MAE 0.0, maxDiff 0.0 on both outputs) against the original split
model using the existing reference tensors.
