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>

packages/runtime-tests/integration/accessibility/screen-reader.test.tsx

@@ -30,6 +30,7 @@ const DIRECTION_LTR = 0;
 function createTestAppContext(): AppContext {
   return {
     exit: () => {},
+    waitUntilRenderFlush: async () => {},
     stdout: process.stdout,
     stderr: process.stderr,
     stdin: process.stdin,