Add X-LANCE multi-stem splitting

Cargo.toml

@@ -25,7 +25,9 @@ cpal = "0.17"
 crossbeam-channel = "0.5"
 eframe = { version = "0.34", features = ["wgpu"] }
 egui = "0.34"
+fs2 = "0.4"
 hound = "3.5"
+libc = "0.2"
 midly = { version = "0.5", default-features = false, features = ["std"] }
 parking_lot = "0.12"
 rfd = "0.17.2"

README.md

@@ -28,6 +28,32 @@ Basic Pitch's CSV represents pitch bends per detected note, while GAW's canonica
 one track-wide bend lane. GAW currently imports note pitch, timing, and velocity and omits those
 per-note bends rather than merging overlapping bends incorrectly.
 
+## X-LANCE stem splitting
+
+GAW can split a materialized audio asset into the eight targets provided by
+[X-LANCE MSR](https://github.com/ModistAndrew/xlance-msr): vocals, guitars, keyboards, bass,
+synthesizers, drums, percussions, and orchestral elements. Select an audio asset and choose
+`STEM SPLITTER…` in its inspector or context menu. The generated assets are added atomically under
+`SPLIT - <original file name>` and retain immutable, content-addressed WAV storage.
+
+The bundled integration currently supports Linux with an NVIDIA CUDA GPU; upstream documents
+inference on an RTX 4090. Create a dedicated Python 3.12 environment before launching GAW:
+
+```sh
+uv venv --python 3.12 .venv-xlance
+uv pip install --python .venv-xlance/bin/python \
+  -r scripts/xlance-requirements-linux.lock
+GAW_XLANCE_PYTHON="$PWD/.venv-xlance/bin/python" cargo run -p gaw-app -- ./projects/my-song
+```
+
+The bundled adapter pins the upstream code and checkpoint revisions. On the first split it clones
+the pinned X-LANCE source and downloads only the selected checkpoints from
+[the official checkpoint repository](https://huggingface.co/chenxie95/xlance-msr-ckpt). A complete
+eight-stem setup uses about 4.3 GB of model weights. Set `GAW_XLANCE_CACHE` to relocate the cache,
+`GAW_XLANCE_REPO` to use an existing checkout, or `GAW_XLANCE` to replace the bundled adapter with a
+compatible executable. Long recordings use a project-local staging area under `.gaw/xlance`; set
+`GAW_XLANCE_TIMEOUT_HOURS` to change the default six-hour job limit.
+
 The product source of truth is [design.md](design.md).
 
 ## Workspace

crates/gaw-app/Cargo.toml

@@ -12,9 +12,11 @@ clap.workspace = true
 crossbeam-channel.workspace = true
 eframe.workspace = true
 egui.workspace = true
+fs2.workspace = true
 gaw-audio = { path = "../gaw-audio" }
 gaw-core = { path = "../gaw-core" }
 gaw-project = { path = "../gaw-project" }
+libc.workspace = true
 rfd.workspace = true
 serde_json.workspace = true
 tempfile.workspace = true