Function validate_parallel_plugin_modes 

pub fn validate_parallel_plugin_modes<L: PluginModeLookup + ?Sized>(
    route: &CompiledRoute,
    registry: &L,
) -> Result<(), String>

Walk a compiled route looking for Effect::Plugin calls nested inside any Effect::Parallel block, and check that each named plugin’s registered mode is safe for parallel execution.

Returns Ok(()) if all plugins inside parallel blocks have safe modes (or the route has no parallel blocks). On failure, returns a ;-separated list of every violation found — running a single pass over the route surfaces all problems at once instead of stopping at the first.