add: absorb soprano_to_rvc as regular subdirectory

Voice conversion pipeline (Soprano TTS → RVC) with Docker support.
Previously tracked as bare gitlink; removed .git/ directories and
absorbed into main repo for unified tracking.

Includes: Soprano TTS, RVC WebUI integration, Docker configs,
WebSocket API, and benchmark scripts.
Updated .gitignore to exclude large model weights (*.pth, *.pt, *.onnx, *.index).
287 files (3.1GB of ML weights properly excluded via gitignore).

soprano_to_rvc/constraints.txt

@@ -0,0 +1,11 @@
+# Constraints to prevent pip from upgrading PyTorch during requirements install
+# The base image rocm/pytorch:rocm6.4_ubuntu22.04_py3.10_pytorch_release_2.5.1 has:
+# - torch 2.5.1+git8420923 (ROCm 6.4 build)
+# - torchvision 0.20.1a0+04d8fc4
+
+# Pin torch to exact version from base image to prevent upgrades
+torch==2.5.1+git8420923
+torchvision==0.20.1a0+04d8fc4
+
+# Prevent standard torchaudio (we'll install ROCm version separately)
+torchaudio