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>

.agents/docs/accessibility-api.md

@@ -1,10 +1,11 @@
 # Accessibility (ARIA + screen-reader) API
 
-How vue-tui exposes ARIA and renders screen-reader (SR) output, and why. Companion to the terse
-blessed entries in [[ink-divergences]] (aria props are camelCase; `renderToString` is layout-only;
-`useWindowSize`); this file keeps the _why_ and the researched / run-verified findings the ledger
-entries deliberately omit, so they are not re-derived expensively. The exported aria types are
-part of the public contract — see [[api-contract]].
+How vue-tui exposes ARIA and renders screen-reader (SR) output, and why. Companion to the
+aria-camelCase blessed entry in [[ink-divergences]]; `renderToString` being layout-only and the
+`useWindowSize` name are now Ink parity (not divergences), and the reactive-refs return shape is
+covered by the shallowRef entry there. This file keeps the _why_ and the researched / run-verified
+findings those entries deliberately omit, so they are not re-derived expensively. The exported aria
+types are part of the public contract — see [[api-contract]].
 
 ## The constraint that shapes everything