Function run_dispatch_loop 

pub fn run_dispatch_loop(dispatch_fn: &str) -> Result<(), &'static str>

Run the dispatch loop directly from Rust, calling PHP via call_user_function.

This is the zero-copy path: serde_json::Value → zval → PHP handler → zval → Value. No JSON encode/decode at all.

dispatch_fn is the name of a PHP function with signature: function(string $method, array $params): array