Plugin Model
CradleXC discovers backend executables from PATH and ~/.cxc/plugins/. A plugin is named cxc-backend-<name>, and CradleXC checks compatibility before using it.
The CLI owns scenario parsing, validation, lowering, compatibility checks and invocation. The plugin owns target-specific file generation. This means a new backend does not require a CradleXC fork or a compiler rebuild.
Use cxc plugin list to confirm that a plugin can be discovered.