feat: cargo-cache composite action skeleton (phase-0 resolution probe)

cargo-cache/action.yml

@@ -0,0 +1,26 @@
+name: 'Cargo CI cache'
+description: 'Per-branch layered Cargo target-dir cache for Gitea Actions runners.'
+inputs:
+  cache-root:
+    description: 'Mount point of the persistent cache volume.'
+    required: false
+    default: '/cache'
+  protected-branches:
+    description: 'Space-separated branches whose cache dirs are never evicted.'
+    required: false
+    default: 'dev main'
+outputs:
+  target-dir:
+    description: 'Resolved CARGO_TARGET_DIR for this run.'
+    value: ${{ steps.probe.outputs.target-dir }}
+runs:
+  using: 'composite'
+  steps:
+    - id: probe
+      shell: bash
+      run: |
+        echo "PROBE-OK: composite action resolved and executed"
+        echo "  cache-root        = ${{ inputs.cache-root }}"
+        echo "  protected-branches= ${{ inputs.protected-branches }}"
+        echo "target-dir=${{ inputs.cache-root }}/probe" >> "$GITHUB_OUTPUT"
+        echo "PROBE_ENV_WRITE=ok" >> "$GITHUB_ENV"