feat(paged): Metal/darwin build availability for llama-cpp-localai-paged

Close the single build-targeting gap the cross-arch audit (ARCH_GENERALITY_AUDIT.md
section 6, item 2) flagged: the paged backend had no Metal/darwin variant and no
metal: capability key, so a Mac user selecting llama-cpp-localai-paged fell back to
default=cpu (a Linux image) that does not run, with no fallthrough to stock llama-cpp.

Mirror exactly how stock llama-cpp does darwin:

- .github/backend-matrix.yml: add the includeDarwin row
  (-metal-darwin-arm64-llama-cpp-localai-paged, arch arm64, lang go) next to the
  stock llama-cpp darwin row.
- backend/index.yaml: add the metal: capability key to the
  llama-cpp-localai-paged meta-backend plus the metal-llama-cpp-localai-paged and
  -development variant entries (URIs match the matrix tag-suffix); add Metal to tags.
- scripts/build/llama-cpp-localai-paged-darwin.sh: new bespoke darwin build,
  a line-for-line mirror of llama-cpp-darwin.sh swapping the paged wrapper dir,
  binary names, ggml-shared-libs dir and output tar. Same CPU_ALL_VARIANTS + Metal
  path (GGML_METAL=ON via the reused llama-cpp Makefile when OS=Darwin; --target ggml
  pulls in ggml-metal via add_dependencies) with LLAMA_PAGED=on.
- Makefile: add backends/llama-cpp-localai-paged-darwin target (+ .NOTPARALLEL).
- .github/workflows/backend_build_darwin.yml: give the paged backend the same
  bespoke darwin build step as stock llama-cpp, share the llama ccache restore (save
  stays stock-only to avoid a same-run key collision), and exclude it from the
  generic build-darwin-go-backend step.
- scripts/changed-backends.js: comment-only - the paged darwin path mapping was
  already present (forward-looking); update the stale "if a metal row is ever added"
  note now that the row exists.

Metal delivers paged-KV only (NVFP4 FP4-MMA is CUDA/Blackwell-only); the GDN/conv
fused ops have no Metal kernel, so a gated-DeltaNet (qwen35) model falls back to the
CPU reference op at runtime - made SAFE by the fused-op backend gate (patch 0030).
This is config; the Metal build runs in CI on the next push and is runtime-tested on
the M4 Mac.

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

.github/workflows/backend_build_darwin.yml

@@ -169,14 +169,14 @@ jobs:
       # invalidates cleanly; restore-keys fall back to the latest entry for the
       # same pin so unchanged TUs stay warm even when the cache is fresh.
       - name: Compute llama.cpp version
-        if: inputs.backend == 'llama-cpp'
+        if: inputs.backend == 'llama-cpp' || inputs.backend == 'llama-cpp-localai-paged'
         id: llama-version
         run: |
           version=$(grep '^LLAMA_VERSION' backend/cpp/llama-cpp/Makefile | head -1 | cut -d= -f2 | cut -d'?' -f1 | tr -d ' ')
           echo "version=${version}" >> "$GITHUB_OUTPUT"
 
       - name: Restore ccache
-        if: inputs.backend == 'llama-cpp'
+        if: inputs.backend == 'llama-cpp' || inputs.backend == 'llama-cpp-localai-paged'
         id: ccache-cache
         uses: actions/cache/restore@v4
         with:
@@ -186,7 +186,7 @@ jobs:
             ccache-llama-${{ runner.arch }}-${{ steps.llama-version.outputs.version }}-
 
       - name: Configure ccache
-        if: inputs.backend == 'llama-cpp'
+        if: inputs.backend == 'llama-cpp' || inputs.backend == 'llama-cpp-localai-paged'
         run: |
           mkdir -p "$HOME/Library/Caches/ccache"
           ccache -M 2G
@@ -230,6 +230,16 @@ jobs:
           make protogen-go
           make backends/llama-cpp-darwin
 
+      # llama-cpp-localai-paged reuses the same bespoke llama-cpp darwin build path
+      # (CPU_ALL_VARIANTS + Metal + otool dylib bundling) via its own wrapper script,
+      # so it gets a dedicated step like stock llama-cpp rather than the generic
+      # build-darwin-go-backend mold.
+      - name: Build ${{ inputs.backend }}-darwin (llama-cpp-localai-paged)
+        if: inputs.backend == 'llama-cpp-localai-paged'
+        run: |
+          make protogen-go
+          make backends/llama-cpp-localai-paged-darwin
+
       - name: Build ds4 backend (Darwin Metal)
         if: inputs.backend == 'ds4'
         run: |
@@ -245,15 +255,20 @@ jobs:
           make backends/privacy-filter-darwin
 
       - name: Build ${{ inputs.backend }}-darwin
-        if: inputs.backend != 'llama-cpp' && inputs.backend != 'ds4' && inputs.backend != 'privacy-filter'
+        if: inputs.backend != 'llama-cpp' && inputs.backend != 'llama-cpp-localai-paged' && inputs.backend != 'ds4' && inputs.backend != 'privacy-filter'
         run: |
           make protogen-go
           BACKEND=${{ inputs.backend }} BUILD_TYPE=${{ inputs.build-type }} USE_PIP=${{ inputs.use-pip }} make build-darwin-${{ inputs.lang }}-backend
 
       - name: ccache stats
-        if: inputs.backend == 'llama-cpp'
+        if: inputs.backend == 'llama-cpp' || inputs.backend == 'llama-cpp-localai-paged'
         run: ccache -s
 
+      # Only stock llama-cpp persists the ccache: both backends share the same
+      # ccache-llama-<arch>-<version>-<run_id> key, so the paged job restores from
+      # the shared prefix (warm) but must NOT also save under the identical key in
+      # the same run (it would collide). The shared upstream TUs stay warm via the
+      # stock save; the paged-only patched TUs are a small recompile.
       - name: Save ccache
         if: inputs.backend == 'llama-cpp' && github.event_name != 'pull_request'
         uses: actions/cache/save@v4