Phase B.5d — TUI vs GTK setup-code audit

[JDonaghy]

Goal

Produce docs/BACKEND_SETUP_AUDIT.md documenting the differences between TUI and GTK init code. This is the diagnostic step that informs the runner-crate API in B.5e — don't start B.5e without it.

Compare

• Event source. TUI: crossterm event::read() blocking, wait_events queue. GTK: Relm4 widget signal callbacks, glib::timeout_add_local drain.
• Backend impl construction. TUI: TuiBackend::new() in main, threading model TBD. GTK: GtkBackend::new() then wrapped in Rc<RefCell<>> shared with widget callbacks.
• Frame loop. TUI: terminal.draw(|f| ...) synchronous. GTK: set_draw_func(da, |da, cr, _, _| ...) callback, fires when GTK queues a redraw.
• Native services. Clipboard (copypasta vs gtk4::Clipboard); file dialogs (TUI does inline, GTK uses native); URL opening (cross-platform crate).
• Per-backend signal-callback boilerplate. Every connect_* in src/gtk/mod.rs::App::init() (key, mouse, scroll, resize, drag, gestures); every match crossterm_event arm in src/tui_main/mod.rs::event_loop().
• App-level state ownership. TUI: Engine plus a few flags lives directly in event_loop. GTK: App struct has 80+ fields, half of them widget refs, half of them rendering caches.
• Cross-backend abstractions already in place. Backend trait, ModalStack, DragState, dispatch_mouse_*, accelerator registry. What's NOT in place that would need to be for the runner.

Output

1. docs/BACKEND_SETUP_AUDIT.md — the comparison.
2. A sketch of the runner-crate API for B.5e (e.g. quadraui_tui::run<App: AppLogic>(...) signature). Ballpark target:

struct MyApp { /* state */ }
impl quadraui::AppLogic for MyApp {
    fn render(&self, ctx: &mut RenderCtx) { /* paint */ }
    fn handle(&mut self, event: UiEvent) -> Reaction { /* dispatch */ }
}

fn main() {
    let app = MyApp::new();
    #[cfg(feature = "gui")] quadraui_gtk::run(app);
    #[cfg(feature = "tui")] quadraui_tui::run(app);
}

The audit decides what RenderCtx looks like, what Reaction carries, what callbacks AppLogic actually needs.

Pickup files

• src/tui_main/mod.rs — run(), event_loop().
• src/gtk/mod.rs — App::init(), the entire view!{} block.
• src/main.rs — where the binaries dispatch to backend.
• examples/ — if there are existing minimal apps already on quadraui (kubeui).

Depends on

• #B5c (the trait must be 100% before the audit; otherwise the audit's "what does the runner need" answer points at gaps that B.5c is going to fill).

PLAN.md "🎯 NEXT FOCUS" section has the full context.

[JDonaghy]

Audit landed at docs/BACKEND_SETUP_AUDIT.md (commit 76b0a51). Closes with the sketched runner-crate API and 7 open design questions for B.5e (#261).