ScreenStack router for push/pop navigation#87
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Type-erased stack of screens (each with its own state, key handler, and view) wired together via a small vtable interface. Distinct from SubProgram (static-typed wrapper for one child model): a ScreenStack holds heterogeneous screens at runtime and supports browser-style push/pop/replace navigation. Screens marked modal are layered on top of the previous screen (centered) — useful for confirm dialogs and command palettes — while non-modal screens hide whatever's beneath. Lifecycle hooks on_enter/on_suspend/on_resume let screens manage per-presentation state.
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Summary
Adds a screen router so apps can push/pop self-contained screens the way a browser navigates pages. Currently every nontrivial Bubble Tea–style app hand-rolls this — now it's a first-class primitive.
How it differs from SubProgram
`SubProgram` is a static-typed wrapper for one specific child model. `ScreenStack` is a runtime-typed stack of heterogeneous screens — login screen, home screen, modal dialog, command palette can all live in the same stack despite being unrelated types.
API
A `Screen` is a thin vtable: `update(key) -> Action`, `view` returning a string, plus optional `deinit`, `on_enter`, `on_suspend`, `on_resume` lifecycle hooks. Actions returned from `update` are: `none`, `pop`, `push(screen)`, `replace(screen)`, `quit`.
`ScreenStack` exposes `push`, `pop`, `replace`, `top`, `depth`, `handleKey`, `view`. Modal screens are rendered centered on top of the screen below; non-modal screens hide whatever's beneath.
Usage
```zig
var stack = zz.ScreenStack.init(allocator);
defer stack.deinit();
try stack.push(login_screen);
// In top-level update:
switch (try stack.handleKey(&ctx, key)) {
.quit => return .quit,
else => {},
}
// In top-level view:
return try stack.view(&ctx, allocator);
```
Test plan