fix(backends): make the opus backend build and package on macOS/Darwin

[localai-bot]

What

Fixes the opus Go backend (realtime WebRTC audio codec) build on macOS / Apple Silicon (Metal). Today the published master-metal-darwin-arm64-opus image ships source only — no opus binary and no libopusshim — because every build/packaging step assumed Linux.

Why

backend/go/opus/{Makefile,run.sh,package.sh} hardcode the Linux shim name (libopusshim.so), LD_LIBRARY_PATH, and Linux-only library bundling, so make -C backend/go/opus build fails on Darwin and golang-darwin.sh packages a binary-less tree (cf. the issue #10267 class of "ships sources, no binary"). The matrix entry under includeDarwin: already exists; only the backend's build was broken.

How

• Makefile — mirror sherpa-onnx: SHIM_EXT=so / dylib on Darwin, build libopusshim.$(SHIM_EXT), and reference the variable in every target + clean. On Darwin the shim is linked with -undefined dynamic_lookup so opus_encoder_ctl resolves from the already globally-loaded libopus (codec.go dlopens libopus RTLD_GLOBAL before the shim) — this avoids baking an absolute Homebrew install-name into the dylib and keeps the packaged shim relocatable.
• run.sh — add a Darwin branch exporting DYLD_LIBRARY_PATH + OPUS_SHIM_LIBRARY=…/libopusshim.dylib (Linux keeps LD_LIBRARY_PATH + .so), matching sherpa-onnx/run.sh. $CURDIR stays quoted.
• package.sh — bundle libopusshim.$(SHIM_EXT) and libopus*.dylib (in addition to libopus.so*) into package/lib, and add a Darwin arch branch (system libs are linked dynamically, no ld.so needed). This makes package/. self-contained on a runtime with no Homebrew.
• backend_build_darwin.yml — brew install/brew link opus + pkg-config (the Makefile/package.sh call pkg-config opus), and cache opus' Cellar dir.

Verification

• bash -n clean on run.sh + package.sh.
• go build ./ and go vet ./ clean in backend/go/opus/ (Go code unchanged).
• Makefile dry-run confirms: Linux → libopusshim.so (+ pkg-config --libs opus); Darwin → libopusshim.dylib -undefined dynamic_lookup.
• A full darwin build cannot run on the Linux dev box — validated in CI.

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