feat: add error boundary with onErrorCaptured to catch descendant errors

Wrap user root component in an InternalErrorBoundary that catches all
descendant errors (setup, render, lifecycle) via onErrorCaptured and
routes them through exit(error). This prevents yoga WASM state corruption
that occurred when errors propagated uncaught during Vue's render phase.

- Add ErrorOverview component for rendering error frames
- Replace try-catch around mount with onErrorCaptured boundary
- Update testing render() to detect early exit errors
- Convert 3 todo tests to active tests (nested setup, unhandledRejection, post-mount)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/runtime-tests/integration/lifecycle/error-handling.test.tsx

@@ -23,12 +23,12 @@ test("render-time throw does not prevent unmount", async () => {
   expect(lastFrame()).toContain("ok");
 
   trigger.value = true;
-  try {
-    await nextTick();
-  } catch {
-    // swallow the render error
-  }
+  // Error boundary catches the render error and routes through exit()
+  await nextTick();
+  await nextTick();
+  await Promise.resolve();
 
+  // After exit(), teardown has run. unmount() should be idempotent/no-throw.
   expect(() => unmount()).not.toThrow();
 });
 
@@ -51,30 +51,57 @@ test("useExit() called with error rejects waitUntilExit", async () => {
   await expect(waitUntilExit()).rejects.toBe(err);
 });
 
-// --- Tests that cannot be ported due to vue-tui runtime limitations ---
+// --- Error boundary tests (previously blocked by yoga WASM crashes) ---
 
-test.todo(
-  "nested component setup error rejects waitUntilExit — " +
-    "errorHandler is installed AFTER mount, so errors during initial mount " +
-    "propagate synchronously instead of routing through exit(err). " +
-    "Additionally, throwing during mount corrupts the WASM yoga tree, " +
-    "making teardown unreliable. Requires runtime-level pre-mount error handling.",
-);
+test("nested component setup error rejects waitUntilExit", async () => {
+  const err = new Error("setup boom nested");
+  const Child = defineComponent(() => {
+    throw err;
+  });
+  const App = defineComponent(() => () => <Child />);
+  await expect(render(App)).rejects.toThrow("setup boom nested");
+});
 
-test.todo(
-  "does not emit unhandledRejection when render exits with an error and waitUntilExit is unused — " +
-    "in vue-tui, setup errors thrown during mount surface via render() rejection " +
-    "and may also produce unhandledRejection events from Vue's internal promise chains " +
-    "before our exitPromise.catch() guard takes effect. Requires engine-level fix.",
-);
+test("does not emit unhandledRejection when render exits with an error and waitUntilExit is unused", async () => {
+  const unhandledErrors: Error[] = [];
+  const handler = (reason: unknown) => {
+    unhandledErrors.push(reason as Error);
+  };
+  process.on("unhandledRejection", handler);
 
-test.todo(
-  "error in component triggered after mount routes through errorHandler — " +
-    "render-function throws during a reactive re-render (post-mount) cause " +
-    "yoga WASM table index out-of-bounds crashes that corrupt the layout engine. " +
-    "The errorHandler is called but the process state is unrecoverable. " +
-    "Requires WASM error isolation or render-phase error recovery in the runtime.",
-);
+  try {
+    const Boom = defineComponent(() => {
+      throw new Error("no-listener boom");
+    });
+    await render(Boom).catch(() => {});
+    // Give a tick for any stray rejections to surface
+    await new Promise((r) => setTimeout(r, 10));
+    expect(unhandledErrors).toHaveLength(0);
+  } finally {
+    process.off("unhandledRejection", handler);
+  }
+});
+
+test("error in component triggered after mount routes through exit", async () => {
+  const trigger = shallowRef(false);
+  const App = defineComponent(() => {
+    return () => {
+      if (trigger.value) throw new Error("post-mount boom");
+      return <Text>ok</Text>;
+    };
+  });
+
+  const { waitUntilExit, lastFrame } = await render(App);
+  expect(lastFrame()).toContain("ok");
+
+  trigger.value = true;
+  // Flush the render + error boundary nextTick + exit microtask
+  await nextTick();
+  await nextTick();
+  await Promise.resolve();
+
+  await expect(waitUntilExit()).rejects.toThrow("post-mount boom");
+});
 
 // --- Ink error validation tests ---