feat(runtime)!: replace useExit() with Ink-aligned useAppContext() (#69)

Ink's `useApp()` returns `{ exit, waitUntilRenderFlush }`. vue-tui previously
exposed only `exit()` via `useExit()` and kept `waitUntilRenderFlush` on the
`TuiApp` handle alone. Align the public surface with Ink: add `useAppContext()`
returning the same pair, and remove `useExit()`.

- thread `waitUntilRenderFlush` into the injected `AppContext` via a hoisted
  impl shared by the `TuiApp` handle and the composable, so both resolve
  identically
- add `useAppContext()`; delete `useExit()`; migrate all call sites, PTY
  fixtures, examples, READMEs and the public-API surface test
- port Ink's two "useApp waitUntilRenderFlush" tests; Ink's third relies on
  React concurrent mode (N/A in Vue)
- rewrite the ink-divergences entry: this is now a *naming* divergence
  (`useAppContext` vs `useApp`, mirroring `createApp` vs `render`), not a
  surface one — and fix the prior wrong claim that Ink's `useApp` returns
  stdin/stdout/stderr

The name is qualified (`useAppContext`, not `useApp`) so it doesn't read as the
Vue application instance (`createApp`/`app.mount`) — the same Vue-native naming
choice vue-tui already makes with `createApp()` vs Ink's `render()`.

BREAKING CHANGE: `useExit()` is removed. Replace `const exit = useExit()` with
`const { exit } = useAppContext()`.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -83,11 +83,11 @@ useInput((input) => {
 
 ## Packages
 
-| Package                                                              | Description                                                                                                                                                                                |
-| -------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
-| [`@vue-tui/runtime`](https://www.npmjs.com/package/@vue-tui/runtime) | The core framework — Vue 3 renderer for the terminal with components (`Box`, `Text`, `Static`, etc.), composables (`useInput`, `useFocus`, `useExit`, etc.), and yoga-based flexbox layout |
-| [`@vue-tui/cli`](https://www.npmjs.com/package/@vue-tui/cli)         | Development tool — `vue-tui dev` starts your app with Vite-powered HMR                                                                                                                     |
-| [`@vue-tui/testing`](https://www.npmjs.com/package/@vue-tui/testing) | Test harness — render in an isolated fake terminal, simulate input, assert output frame by frame                                                                                           |
+| Package                                                              | Description                                                                                                                                                                                      |
+| -------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
+| [`@vue-tui/runtime`](https://www.npmjs.com/package/@vue-tui/runtime) | The core framework — Vue 3 renderer for the terminal with components (`Box`, `Text`, `Static`, etc.), composables (`useInput`, `useFocus`, `useAppContext`, etc.), and yoga-based flexbox layout |
+| [`@vue-tui/cli`](https://www.npmjs.com/package/@vue-tui/cli)         | Development tool — `vue-tui dev` starts your app with Vite-powered HMR                                                                                                                           |
+| [`@vue-tui/testing`](https://www.npmjs.com/package/@vue-tui/testing) | Test harness — render in an isolated fake terminal, simulate input, assert output frame by frame                                                                                                 |
 
 ## Examples
 
@@ -116,7 +116,7 @@ useInput((input) => {
 | `useInput(handler, opts?)` | Handle keyboard input — receives `(input, key)` with modifier and arrow key detection |
 | `useFocus(opts?)`          | Component-level focus — returns `{ isFocused, focus }`                                |
 | `useFocusManager()`        | App-level focus control — `focusNext()`, `focusPrevious()`, `focus(id)`               |
-| `useExit()`                | Programmatic app exit — returns `exit(error?)`                                        |
+| `useAppContext()`          | App context — `{ exit(error?), waitUntilRenderFlush() }`                              |
 | `useTerminalSize()`        | Reactive terminal dimensions — `{ columns, rows }`                                    |
 | `useStdin()`               | Access stdin stream and raw mode control                                              |
 | `useStdout()`              | Write directly to stdout                                                              |