fix(backends): darwin/metal support for supertonic

[localai-bot]

What

Adds macOS (darwin-arm64 / Metal) build and runtime support to the supertonic Go TTS backend, which dlopens ONNX Runtime via purego (yalue/onnxruntime_go).

The backend's runtime/packaging scripts were Linux-only. This applies the same purego .dylib / DYLD_LIBRARY_PATH fix that PR #10481 landed for 15 other ONNX/purego backends (sherpa-onnx, silero-vad, etc.) - supertonic was omitted from that PR - and wires the darwin CI build, resolving the inconsistency where backend/index.yaml advertised metal for supertonic but no includeDarwin matrix entry actually built the image.

The 4-layer dylib fix (mirroring sherpa-onnx)

• run.sh: on darwin export DYLD_LIBRARY_PATH (keep LD_LIBRARY_PATH for Linux), point ONNXRUNTIME_LIB_PATH at libonnxruntime.dylib, and guard the ld.so exec path to Linux only (macOS has no ld.so loader).
• package.sh: recognize Darwin (elif [ $(uname -s) = "Darwin" ]) instead of erroring out; the bundled .dylib is resolved via DYLD_LIBRARY_PATH, with no glibc/ld.so to bundle.
• helper.go: platform-native default library extension (.dylib on runtime.GOOS == "darwin", .so elsewhere) for the last-resort dlopen fallback. Build-tag-free, compiles on both OSes.
• Makefile: no change needed - it already detects Darwin/osx/arm64 and stages the per-OS ONNX Runtime tarball (backend-assets/lib copies sources/onnxruntime/lib/*, which is .dylib on macOS), exactly like sherpa-onnx.

CI / gallery wiring

• .github/backend-matrix.yml: add the -metal-darwin-arm64-supertonic entry to includeDarwin (lang: "go", build-type: "metal").
• backend/index.yaml: declare metal capabilities on the supertonic and supertonic-development entries, and add the concrete metal-supertonic / metal-supertonic-development child entries (mirroring metal-sherpa-onnx).

Verification

• python3 -c "import yaml; yaml.safe_load(...)" passes for both backend/index.yaml and .github/backend-matrix.yml.
• The Go package still compiles on Linux (go build ./... after make protogen-go); go vet is clean.
• The darwin CI build (this matrix entry) validates the image builds on Apple Silicon. Runtime TTS inference on Apple Silicon is pending (no local M-series hardware available to the author).

Assisted-by: Claude:opus-4.8 [Claude Code]