Function lumen_app_on_click 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn lumen_app_on_click(
    app: *mut LumenApp,
    id: *const c_char,
    cb: Option<LumenClickFn>,
    user_data: *mut c_void,
) -> LumenStatus

Register an id-scoped native click handler (ABI 0.3).

cb fires once per click on the element whose LumenId equals id, routed by the runtime - no main.lmn forwarding boilerplate and no per-embedder dispatch table over the global on_click(id) hook. A second registration for the same id replaces the first. Must be called before lumen_app_run / lumen_app_run_headless.

The handler fires on the Lumen tick thread; user_data carries the same Send/Sync contract as lumen_app_expose’s. The native routing coexists with any script-side on_click(id) handler - both observe the same click.