docs: clarify cross-build setup for linux/arm64 in CI example

Add an inline YAML comment and a note explaining that the linux/arm64
matrix entry uses cross-building via TARGET_ARCH env var on an x64
runner, and that engines with native compilation steps may need actual
arm64 runners or QEMU emulation.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: GitHub Copilot

docs/integration-guide.md

@@ -935,7 +935,7 @@ jobs:
           - os: ubuntu-latest
             platform: linux
             arch: x64
-          - os: ubuntu-latest
+          - os: ubuntu-latest # Cross-build: uses TARGET_ARCH env var, not native arm64
             platform: linux
             arch: arm64
           - os: macos-latest
@@ -978,6 +978,8 @@ jobs:
 
 Each matrix job builds for its target platform (using `TARGET_PLATFORM` and `TARGET_ARCH` environment variables) and uploads a named tarball. The result is a single GitHub Release with one artifact per platform.
 
+> **Note:** This example uses cross-building — the `linux/arm64` job runs on an x64 runner and relies on `TARGET_ARCH` to select the correct platform-specific binaries at build time rather than compiling natively. This works well for engines that bundle pre-built binaries or use interpreted runtimes. If your engine includes native compilation steps (e.g., building C/C++ extensions or Rust crates for arm64), you will need either an actual arm64 runner or a QEMU/cross-toolchain emulation setup.
+
 ### What to Include in the Release
 
 Your release artifact must include `engine.yaml` and everything it references. The `entrypoint` in `engine.yaml` is resolved as a relative path, so any files or directories it points to need to be in the tarball alongside it. For example, if your entrypoint is `node dist/index.js`, then `dist/index.js` (and any of its runtime dependencies) must be present.