Pin transition commit-on-handled-error behavior from the #3107 router thread

A source write entangled with downstream async that rejects must still
commit — the error boundary handling the rejection is the transition
resolving, not it failing. This pin came out of the router navigation
investigation and was never landed; tests only, no changeset.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: ryansolid

packages/signals/tests/router-transition-error-commit.test.ts

@@ -0,0 +1,61 @@
+import {
+  createEffect,
+  createErrorBoundary,
+  createMemo,
+  createRenderEffect,
+  createRoot,
+  createSignal,
+  flush
+} from "../src/index.js";
+
+afterEach(() => flush());
+
+it("commits a source write and its effect when a downstream async error is handled", async () => {
+  const error = new Error("lazy route failed");
+  let reject!: (error: unknown) => void;
+  const [route, setRoute] = createSignal("/", { ownedWrite: true });
+  let rendered = "";
+  let external = "/";
+
+  createRoot(() => {
+    const lazyRoute = createMemo(() => {
+      if (route() === "/") return "home";
+      return new Promise<string>((_, fail) => (reject = fail));
+    });
+    const boundary = createErrorBoundary(
+      () => lazyRoute(),
+      caught => {
+        expect(caught()).toBe(error);
+        return "error";
+      }
+    );
+    createRenderEffect(boundary, value => {
+      rendered = value;
+    });
+    createEffect(
+      route,
+      value => {
+        external = value;
+      },
+      { defer: true }
+    );
+  });
+
+  flush();
+  expect(rendered).toBe("home");
+
+  setRoute("/plugins");
+  flush();
+  expect(route()).toBe("/");
+  expect(rendered).toBe("home");
+  expect(external).toBe("/");
+
+  reject(error);
+  await Promise.resolve();
+  await Promise.resolve();
+  flush();
+
+  expect(rendered).toBe("error");
+  expect(route()).toBe("/plugins");
+  expect(external).toBe("/plugins");
+});