docs(runtime): finish the useWindowSize rename in docs; tidy contract guards (#164)

Follow-up cleanup for the 7 confirmed findings from a review of #163. The
dominant theme: #163 hard-renamed the public composable useTerminalSize ->
useWindowSize (no alias) but left stale references to the dead name in
user-facing docs.

- README.md + packages/runtime/README.md: the composable tables named the
  removed `useTerminalSize()` (root README even framed the sole real export
  `useWindowSize` as an "Ink-compat alias" — now inverted). Point both at
  `useWindowSize()`.
- .agents/docs/ink-divergences.md: two vue-tui-side references to
  `useTerminalSize` (the shallowRef "object of refs" example and the
  "composables throw outside a render tree" list) -> `useWindowSize`. The
  Ink-side `useWindowSize -> WindowSize` naming example is left unchanged.
- .agents/docs/accessibility-api.md: the intro cited three "blessed entries"
  but only aria-camelCase is one; `renderToString` layout-only and the
  `useWindowSize` name are now Ink parity, not divergences. Reword.
- .agents/docs/api-contract.md: tighten the `/internal` wording — the test
  does assert one tripwire on `/internal`, so "not covered by
  public-api.test.ts" was imprecise.
- public-api.test.ts / render-to-string.test.tsx: the public renderToString
  dropped the `isScreenReaderEnabled` option but (unlike the sibling
  `ScreenReaderOptions` type) had no compile-time guard. Replace an obscure,
  fmt-fragile type-indexing guard with a readable call-site `@ts-expect-error`
  in render-to-string.test.tsx; re-adding the option to the public
  RenderToStringOptions makes the directive unused and fails `tsc --noEmit`.
- Rename terminal-size.test.tsx / .sequential.test.tsx ->
  window-size.test.tsx / .sequential.test.tsx to match the migrated symbol.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

@@ -111,20 +111,20 @@ useInput((input) => {
 
 ## Composables (Hooks)
 
-| Composable                 | Description                                                                            |
-| -------------------------- | -------------------------------------------------------------------------------------- |
-| `useInput(handler, opts?)` | Handle keyboard input — receives `(input, key)` with modifier and arrow key detection  |
-| `usePaste(handler, opts?)` | Handle bracketed paste — receives the pasted `text` as a single event                  |
-| `useFocus(opts?)`          | Component-level focus — returns `{ isFocused, focus }`                                 |
-| `useFocusManager()`        | App-level focus control — `focusNext()`, `focusPrevious()`, `focus(id)`                |
-| `useApp()`                 | App lifecycle — `{ exit(error?), waitUntilRenderFlush() }`                             |
-| `useTerminalSize()`        | Reactive terminal dimensions — `{ columns, rows }` (Ink-compat alias: `useWindowSize`) |
-| `useStdin()`               | Access stdin stream and raw mode control                                               |
-| `useStdout()`              | Write directly to stdout                                                               |
-| `useStderr()`              | Write directly to stderr                                                               |
-| `useBoxMetrics(ref)`       | Measure a `<Box>` via a template ref — reactive `{ width, height, left, top }`         |
-| `useCursor()`              | Control the terminal cursor — `setCursorPosition(pos)` in output coordinates           |
-| `useAnimation(opts?)`      | Frame-based animation driver — reactive `{ frame, time, delta }` + `reset()`           |
+| Composable                 | Description                                                                           |
+| -------------------------- | ------------------------------------------------------------------------------------- |
+| `useInput(handler, opts?)` | Handle keyboard input — receives `(input, key)` with modifier and arrow key detection |
+| `usePaste(handler, opts?)` | Handle bracketed paste — receives the pasted `text` as a single event                 |
+| `useFocus(opts?)`          | Component-level focus — returns `{ isFocused, focus }`                                |
+| `useFocusManager()`        | App-level focus control — `focusNext()`, `focusPrevious()`, `focus(id)`               |
+| `useApp()`                 | App lifecycle — `{ exit(error?), waitUntilRenderFlush() }`                            |
+| `useWindowSize()`          | Reactive terminal dimensions — `{ columns, rows }`                                    |
+| `useStdin()`               | Access stdin stream and raw mode control                                              |
+| `useStdout()`              | Write directly to stdout                                                              |
+| `useStderr()`              | Write directly to stderr                                                              |
+| `useBoxMetrics(ref)`       | Measure a `<Box>` via a template ref — reactive `{ width, height, left, top }`        |
+| `useCursor()`              | Control the terminal cursor — `setCursorPosition(pos)` in output coordinates          |
+| `useAnimation(opts?)`      | Frame-based animation driver — reactive `{ frame, time, delta }` + `reset()`          |
 
 ## Testing