feat(backends): darwin/Metal build for the privacy-filter backend (timeboxed try)

The privacy-filter.cpp engine is already Metal-capable on Apple Silicon: it pulls
ggml and never forces GGML_METAL=OFF, and ggml defaults Metal ON on Apple, so a
plain Darwin build is Metal-enabled. grpc++/protobuf resolve from Homebrew via
find_package(... CONFIG). It just had no darwin build path - the existing
package.sh and run.sh are Linux-only and there was no make target / workflow step.

Adds the bespoke darwin path, modeled on the ds4 one:
- scripts/build/privacy-filter-darwin.sh: native make grpc-server, otool -L dylib
  bundling, create-oci-image (no Linux package.sh).
- Makefile: backends/privacy-filter-darwin target (+ .NOTPARALLEL).
- .github/workflows/backend_build_darwin.yml: gated build step for privacy-filter.
- scripts/changed-backends.js: inferBackendPathDarwin special-case -> backend/cpp.
- .github/backend-matrix.yml: includeDarwin entry (lang go, like ds4/llama-cpp).
- backend/index.yaml: metal: capability + metal-privacy-filter(-development) entries.
- backend/cpp/privacy-filter/run.sh: DYLD_LIBRARY_PATH branch on Darwin.

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

scripts/build/privacy-filter-darwin.sh

@@ -0,0 +1,58 @@
+#!/bin/bash
+# Darwin/Metal build for the privacy-filter backend. Mirrors ds4-darwin.sh:
+# native make of the single grpc-server, otool -L dylib bundling, then assemble
+# an OCI tar that `local-ai backends install` can consume.
+#
+# privacy-filter.cpp pulls ggml, which defaults GGML_METAL=ON on Apple - the
+# engine's CMake never forces it off, so a plain Darwin build is Metal-enabled.
+# grpc++/protobuf are resolved from Homebrew via find_package(... CONFIG).
+set -ex
+
+IMAGE_NAME="${IMAGE_NAME:-localai/privacy-filter-darwin}"
+
+pushd backend/cpp/privacy-filter
+make grpc-server
+popd
+
+mkdir -p build/darwin
+mkdir -p build/darwin/lib
+mkdir -p backend-images
+
+cp -rf backend/cpp/privacy-filter/grpc-server build/darwin/
+cp -rf backend/cpp/privacy-filter/run.sh      build/darwin/
+
+# Apple Silicon: pick up Homebrew-installed protobuf utf8_validity if present
+# (same as ds4-darwin.sh - it is a transitive dep otool may not surface).
+if [[ "$(uname -s)" == "Darwin" && "$(uname -m)" == "arm64" ]]; then
+    ADDITIONAL_LIBS=${ADDITIONAL_LIBS:-$(ls /opt/homebrew/Cellar/protobuf/**/lib/libutf8_validity*.dylib 2>/dev/null)}
+else
+    ADDITIONAL_LIBS=${ADDITIONAL_LIBS:-""}
+fi
+for file in $ADDITIONAL_LIBS; do
+    cp -rfv "$file" build/darwin/lib
+done
+
+# Walk dylibs via otool -L and bundle anything that isn't a system framework.
+for file in build/darwin/grpc-server; do
+    LIBS="$(otool -L "$file" | awk 'NR > 1 { system("echo " $1) } ' | xargs echo)"
+    for lib in $LIBS; do
+        if [[ "$lib" == *.dylib ]] && [[ -e "$lib" ]]; then
+            cp -rvf "$lib" build/darwin/lib
+        fi
+    done
+done
+
+echo "Bundled libraries:"
+ls -la build/darwin/lib
+
+# Build an OCI image tar (with manifest.json) that `local-ai backends install`
+# can consume - mirrors ds4-darwin.sh.
+PLATFORMARCH="${PLATFORMARCH:-darwin/arm64}"
+
+./local-ai util create-oci-image \
+        build/darwin/. \
+        --output ./backend-images/privacy-filter.tar \
+        --image-name "$IMAGE_NAME" \
+        --platform "$PLATFORMARCH"
+
+rm -rf build/darwin

scripts/changed-backends.js

@@ -71,6 +71,11 @@ function inferBackendPathDarwin(item) {
   if (item.backend === "ds4") {
     return `backend/cpp/ds4/`;
   }
+  // privacy-filter is C++ too (built via `make backends/privacy-filter-darwin`);
+  // same lang=go-for-runner convention, source under backend/cpp.
+  if (item.backend === "privacy-filter") {
+    return `backend/cpp/privacy-filter/`;
+  }
   if (!item.lang) {
     return `backend/python/${item.backend}/`;
   }