fix(runtime): keep text-measure helpers internal, matching Ink (#68)

Ink keeps its `measure-text` module internal and never re-exports it. vue-tui
exported `measureText` / `measureTextNatural` from the public index under the
mistaken belief — stated verbatim in commit 0e7d775's own message — that doing
so "matched Ink's public API". It does not; Ink keeps that module internal. A
later design doc then rationalized the leak post-hoc as an intentional
divergence. It was neither intentional nor a divergence — it was a mistake.

Align with Ink:
- Drop both from the public index. `measureTextNatural` stays as an internal
  helper (yoga.ts uses it). `measureText` had zero production callers (yoga uses
  `wrapText` + `measureTextNatural`, never `measureText`) and is removed.
- Integration tests that used `measureText(stripAnsi(x), 9999).width` as a
  line-width helper now use `stringWidth(stripAnsi(x))` directly.
- public-api.test.ts gains a regression test asserting neither is exported.
- Remove the now-obsolete entry from .agents/docs/ink-divergences.md.

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

packages/runtime-tests/integration/components/borders.test.tsx

@@ -1,8 +1,9 @@
 import { defineComponent, shallowRef, nextTick } from "vue";
 import { test } from "vite-plus/test";
 import { render } from "@vue-tui/testing";
-import { Box, Text, measureText } from "@vue-tui/runtime";
+import { Box, Text } from "@vue-tui/runtime";
 import stripAnsi from "strip-ansi";
+import stringWidth from "string-width";
 
 // single node — full width box
 test("single node - full width box", async ({ expect }) => {
@@ -977,7 +978,7 @@ test("arrow border on narrow box does not overflow", async ({ expect }) => {
   );
   const frame = lastFrame()!;
   for (const line of frame.split("\n")) {
-    expect(measureText(stripAnsi(line), 9999).width).toBeLessThanOrEqual(3);
+    expect(stringWidth(stripAnsi(line))).toBeLessThanOrEqual(3);
   }
   const stripped = stripAnsi(frame);
   expect(stripped.split("\n")[0]).toContain("↘");