CI: cache MLX build artifacts and bump to Node.js 24 actions

Cache ~/Library/Caches/emily (MLX cmake build + NIF object files)
to avoid rebuilding MLX from source on every CI run (~6 min).
Bump checkout v4→v6 and cache v4→v5 to silence Node.js 20
deprecation warnings.

Author: ausimian

.github/workflows/ci.yml

@@ -22,7 +22,7 @@ jobs:
     env:
       MIX_ENV: test
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
         with:
           submodules: recursive
 
@@ -33,25 +33,34 @@ jobs:
           version-type: strict
 
       - name: Cache deps
-        uses: actions/cache@v4
+        uses: actions/cache@v5
         with:
           path: deps
           key: deps-${{ runner.os }}-${{ hashFiles('mix.lock') }}
           restore-keys: deps-${{ runner.os }}-
 
       - name: Cache build
-        uses: actions/cache@v4
+        uses: actions/cache@v5
         with:
           path: _build
           key: build-${{ runner.os }}-${{ hashFiles('mix.lock', 'c_src/**', 'Makefile', '.tool-versions', '.gitmodules') }}
           restore-keys: build-${{ runner.os }}-
 
+      # MLX cmake build (~6 min from source) and NIF object files.
+      # Keyed by the vendored MLX submodule commit + NIF sources.
+      - name: Cache MLX and NIF objects
+        uses: actions/cache@v5
+        with:
+          path: ~/Library/Caches/emily
+          key: mlx-${{ runner.os }}-${{ hashFiles('.gitmodules', 'c_src/**', 'Makefile') }}
+          restore-keys: mlx-${{ runner.os }}-
+
       # The Bumblebee cache holds tiny-random HuggingFace fixtures
       # downloaded by the conformance suite (~3 MB across 7 repos).
       # Cached by conformance test content so new fixtures invalidate;
       # restore-keys lets unrelated changes reuse the existing cache.
       - name: Cache Bumblebee fixtures
-        uses: actions/cache@v4
+        uses: actions/cache@v5
         with:
           path: ~/Library/Caches/bumblebee
           key: bumblebee-${{ runner.os }}-${{ hashFiles('test/emily/conformance/**') }}