feat(backends): add darwin/metal build for liquid-audio (#10486)

* feat(backends): add darwin/metal build for liquid-audio

Wire the already-MPS-ready liquid-audio backend (it ships
requirements-mps.txt) into the darwin CI matrix and the gallery so
metal-darwin-arm64 images are built and selectable.

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
Assisted-by: Claude:opus-4.8 [Claude Code]

* ci(liquid-audio): trigger darwin build via requirements-mps note

The changed-backends path filter only builds a backend when a file under
its directory changes. The metal wiring lived in index.yaml + the matrix,
so the darwin job was skipped. Add a documenting comment to the MPS
requirements so CI actually exercises the darwin build.

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
Assisted-by: Claude:opus-4.8 [Claude Code]

* fix(liquid-audio): guard uv-only --index-strategy for the pip/darwin path

Same fix as trl: the darwin/MPS build installs with pip (USE_PIP=true), which
rejects the uv-only --index-strategy flag and failed the darwin backend build.
Add it only on the uv path; Linux/CUDA resolution is unchanged.

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
Assisted-by: Claude:opus-4.8 [Claude Code]

---------

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
Co-authored-by: Ettore Di Giacinto <mudler@localai.io>

.github/backend-matrix.yml

@@ -4974,6 +4974,9 @@ includeDarwin:
   - backend: "kitten-tts"
     tag-suffix: "-metal-darwin-arm64-kitten-tts"
     build-type: "mps"
+  - backend: "liquid-audio"
+    tag-suffix: "-metal-darwin-arm64-liquid-audio"
+    build-type: "mps"
   - backend: "piper"
     tag-suffix: "-metal-darwin-arm64-piper"
     build-type: "metal"