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

[localai-bot]

From the registry sweep: the gallery has metal-ds4 / metal-ds4-development entries, but no metal-darwin-arm64-ds4 image is ever published, so those entries dangle and ds4 is un-installable on Apple Silicon.

Root cause: ds4 was never listed in the darwin matrix (includeDarwin in .github/backend-matrix.yml), even though the build recipe exists - make backends/ds4-darwin, special-cased in backend_build_darwin.yml.

Changes

1. Add ds4 to the darwin matrix, mirroring the llama-cpp form (also special-cased, so just lang: go):

     - backend: "ds4"
       tag-suffix: "-metal-darwin-arm64-ds4"
       lang: "go"

2. Fix inferBackendPathDarwin in scripts/changed-backends.js to map ds4 → backend/cpp/ds4/ (like llama-cpp). ds4 is C++ but its matrix entry carries lang: go, so the generic inference would return backend/go/ds4/ (nonexistent) and ds4s darwin build would only ever run on a release (FORCE_ALL), never incrementally when backend/cpp/ds4/ changes.

How darwin builds trigger (important)

changed-backends.js only selects a backends darwin build when a changed file is under its source prefix, or on a tag/release push (FORCE_ALL=true rebuilds everything). So after this merges, the metal-darwin-arm64-ds4 image publishes at the next release (or when backend/cpp/ds4/ changes). The gallery metal-ds4 entries then resolve.

Related finding (separate, not fixed here)

sherpa-onnx is already in the darwin matrix (added by #10275) but its metal image was never published - because #10275 only added the matrix entry + touched backend/python/speaker-recognition/requirements-mps.txt, so the path filter triggered speaker-recognitions darwin build (its master- image published) but not sherpa-onnx (no backend/go/sherpa-onnx/ change). sherpa-onnx is correctly configured and will build at the next release; no fix needed.

Notes

• Validated: backend-matrix.yml parses, includeDarwin now lists ds4; changed-backends.js passes a module syntax check.
• Actual metal compilation only runs on the macOS CI runner.

Assisted-by: claude:claude-opus-4-8 [Claude Code]