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>

packages/runtime-tests/integration/public-api.test.ts

@@ -71,3 +71,6 @@ test("does not expose the screen-reader linearizer publicly (Ink keeps it intern
 // It DOES type-check from `/internal`, which the runtime guard above already proves is the home.
 // @ts-expect-error - ScreenReaderOptions is exported only from @vue-tui/runtime/internal
 export type _ScreenReaderOptionsIsInternalOnly = import("@vue-tui/runtime").ScreenReaderOptions;
+
+// The parallel guard for the public `renderToString`'s dropped `isScreenReaderEnabled` OPTION lives
+// in render-to-string.test.tsx (a call-site `@ts-expect-error`), next to the renderToString tests.

packages/runtime-tests/integration/render-to-string.test.tsx

@@ -42,6 +42,18 @@ describe("renderToString", () => {
     expect(output).toBe("test");
   });
 
+  // Contract guard: `isScreenReaderEnabled` is INTERNAL-only — the public `renderToString` must
+  // reject it at the type level (SR rendering goes through `renderToStringWithScreenReader` in
+  // `@vue-tui/runtime/internal`). If the option is ever re-added to the public `RenderToStringOptions`,
+  // the `@ts-expect-error` below goes unused and `tsc --noEmit` fails. (At runtime the unknown option
+  // is harmlessly ignored — only `columns` is read — so the frame still renders.)
+  test("public renderToString rejects the internal isScreenReaderEnabled option (type-level)", () => {
+    const App = defineComponent(() => () => <Text>x</Text>);
+    // @ts-expect-error - isScreenReaderEnabled is internal-only (use renderToStringWithScreenReader from /internal)
+    const output = renderToString(App, { isScreenReaderEnabled: true });
+    expect(output).toBe("x");
+  });
+
   test("rethrows component errors after cleanup", () => {
     const App = defineComponent(() => {
       throw new Error("boom");