Function prepare_export_install 

pub fn prepare_export_install(
    manifest_json: &str,
    registry: &PluginRegistry,
) -> Result<Manifest, String>

Validate an incoming export-plugin manifest end to end and return it ready to register. Like content, an export adapter is declarative (no wasm), so the signature is verified over the manifest alone. Runs: parse → schema validation (which checks the export config) → export-only gate → signature → not-already-installed.