Fix MPS/Metal crash: embed Metal library at build time + add comfy-action CI

[Copilot]

On macOS Apple Silicon, ace-qwen3 aborts immediately (exit -6 / SIGABRT) because ggml JIT-compiles embedded Metal shader source at runtime via newLibraryWithSource:, and certain macOS/Xcode SDK combinations reject that source with MTLLibraryErrorDomain template type-mismatch errors in kernel_col2im_1d / kernel_snake — before any model file is touched.

Fix

-DGGML_METAL_EMBED_LIBRARY=ON added to cmake Metal flags (both nodes.py and install.py):

# Before
"metal": [],  # Metal is auto-enabled on macOS by ggml

# After
"metal": ["-DGGML_METAL_EMBED_LIBRARY=ON"],  # pre-compile shaders at build time

ggml then compiles the Metal shaders at cmake build time (xcrun metal) and embeds the resulting binary .metallib directly in the executable — no runtime JIT, no runtime shader errors.

Additional changes

• Better error surfacing — when ace-qwen3 fails on macOS and stderr contains MTLLibraryErrorDomain, generate() now appends a targeted message explaining the cause and directing the user to rebuild or switch to backend=cpu.
• New CI workflow (.github/workflows/ci-mps.yml) using Comfy-Org/comfy-action@main on macos-latest (GitHub-hosted Apple Silicon runner):
  1. Installs ComfyUI via comfy-cli into the workspace root (required for comfy-action's python main.py)
  2. Copies this node into custom_nodes/ and builds binaries with the embed flag
  3. Metal smoke test — runs ace-qwen3 at startup against a nonexistent model; exit 134 (SIGABRT) means Metal JIT failed, catches the regression without any model downloads
  4. Downloads minimum GGUF set (0.6B LM + Q4_K_M DiT, cached between runs)
  5. Runs acestep-cpp-text2music.json end-to-end via Comfy-Org/comfy-action — a full workflow failure would have been the first visible symptom of the original bug

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