Macro smb_dispatch 

macro_rules! smb_dispatch {
    ( $( $variant:ident => $handler:ty ; )* ) => { ... };
}

Static, monomorphic dispatch to command handlers. Generated separately from the request table so the two concerns stay decoupled; the _ arm is unreachable now that every decodable command has a handler, but is kept so a future table-only addition still compiles.