fix(dev): repair the HMR client and keep islands alive across updates

The HMR client script was dead in the browser. HMR_CLIENT_JS is a
TypeScript template literal, so the regex [ \t\r\n] inside it was expanded
into real control characters, producing a regex literal containing a raw
newline — a syntax error that took the whole script down with "Invalid
regular expression: missing /". It now uses \s, and a test asserts the
emitted client parses and holds no control characters inside regex
literals; that test fails if the bug is reintroduced.

HMR also corrupted CSP nonces. A document's nonce is fixed at load, but
morph copied attributes from freshly fetched HTML, overwriting the live
nonce with one the browser will not honour. syncAttrs now leaves nonce
alone, and nodes moved across are re-stamped with the live nonce.

Islands vanished on every HMR update: morph puts the server placeholder
back over the mounted island. The island runtime now remounts on
wrnexus:hmr-updated. Remounting swaps the container for a bare clone —
re-rendering the existing root is a no-op once HMR has wiped the DOM
externally, and unmounting throws asynchronously because the nodes React
wants to remove are already gone.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

packages/react/test/island-hmr.test.ts

@@ -70,3 +70,25 @@ test("repeated remounts stay at one root", async () => {
 
   expect(islandRootCount()).toBe(1);
 });
+
+test("remount re-renders in place instead of creating a second root", async () => {
+  const window = domWith(marker);
+  const loader = async () => ({ default: () => createElement("span", null, "v1") });
+
+  await act(async () => {
+    await mountIslands(host(window), { loader });
+  });
+  expect(islandRootCount()).toBe(1);
+
+  // Simulate HMR morphing server markup back over the mounted island: React's
+  // rendered DOM is gone, so unmounting it would throw.
+  const el = window.document.querySelector("[data-wrn-island]")!;
+  el.innerHTML = "";
+
+  await act(async () => {
+    await remountIslands(host(window), { loader });
+  });
+
+  expect(islandRootCount()).toBe(1);
+  expect(window.document.body.textContent).toContain("v1");
+});

packages/react/test/runtime-source.test.ts

@@ -23,3 +23,11 @@ test("threads the development flag into the mount options", () => {
 test("encodes the island name before using it as a URL path segment", () => {
   expect(getIslandRuntime(false)).toContain("encodeURIComponent");
 });
+
+test("the runtime remounts islands after an HMR update", () => {
+  // HMR morphs the server placeholder over the mounted island, discarding the
+  // React tree; without this the island silently disappears on every edit.
+  const source = getIslandRuntime(true);
+  expect(source).toContain("wrnexus:hmr-updated");
+  expect(source).toContain("__wrnexusRemountIslands");
+});