[codex] add arm64 smoke test workflow

.github/workflows/arm64-smoke.yml

@@ -0,0 +1,51 @@
+name: ARM64 Smoke Test
+
+on:
+  pull_request:
+    branches: [main]
+    paths:
+      - ".github/workflows/**"
+      - ".cargo/**"
+      - "Cargo.toml"
+      - "Cargo.lock"
+      - "build.rs"
+      - "install.sh"
+      - "src/**"
+  workflow_dispatch:
+
+env:
+  CARGO_TERM_COLOR: always
+
+jobs:
+  arm64-linux:
+    name: Native ARM64 Linux Smoke Test
+    runs-on: ubuntu-24.04-arm
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Setup Rust toolchain
+        uses: dtolnay/rust-toolchain@master
+        with:
+          toolchain: stable
+
+      - name: Set up cargo cache
+        uses: Swatinem/rust-cache@v2
+        with:
+          key: ubuntu-24.04-arm-smoke
+
+      - name: Build ARM64 binary
+        run: cargo build --release
+
+      - name: Package expected release asset
+        run: |
+          mkdir -p dist
+          cp target/release/popcorn-cli dist/popcorn-cli
+          chmod +x dist/popcorn-cli
+          tar -czf popcorn-cli-linux-aarch64.tar.gz -C dist popcorn-cli
+
+      - name: Verify packaged binary runs
+        run: |
+          mkdir -p smoke
+          tar -xzf popcorn-cli-linux-aarch64.tar.gz -C smoke
+          ./smoke/popcorn-cli --version