ci(darwin): build the ds4 backend for darwin/arm64 (metal) (#10303)

The gallery has metal-ds4 / metal-ds4-development entries, and the build
recipe exists (make backends/ds4-darwin, special-cased in
backend_build_darwin.yml), but ds4 was never listed in the darwin matrix,
so no metal-darwin-arm64-ds4 image was ever published and the entries
dangled.

- Add ds4 to the darwin matrix (includeDarwin), mirroring the llama-cpp
  form (the reusable workflow builds it via 'make backends/ds4-darwin').
- Fix inferBackendPathDarwin in scripts/changed-backends.js to map ds4 to
  backend/cpp/ds4/ (like llama-cpp): ds4 is C++ but the matrix entry carries
  lang=go, so without this its darwin build would only ever run on a release
  (FORCE_ALL), never incrementally when backend/cpp/ds4 changes.

sherpa-onnx and speaker-recognition are already in the darwin matrix on
master and are not changed here.

Assisted-by: claude: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

@@ -4475,3 +4475,6 @@ includeDarwin:
   - backend: "speaker-recognition"
     tag-suffix: "-metal-darwin-arm64-speaker-recognition"
     build-type: "mps"
+  - backend: "ds4"
+    tag-suffix: "-metal-darwin-arm64-ds4"
+    lang: "go"