feat(islands): serve the island runtime in dev, prod, and static builds

Adds /__wrnexus/islands.js (the bootstrap) and the /__wrnexus/island/
prefix (mount runtime, island bundles, shared chunks) to all three
serving paths.

Dev reuses the browserArtifactPaths registry pattern from pipeline.ts.
Prod mirrors the clientModulesDir handler, including its filename
allowlist, so island names cannot escape the output directory.

The bootstrap is inert without a data-wrn-island marker, so island-free
pages still download nothing.

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

packages/dev-server/src/pipeline.ts

@@ -57,6 +57,7 @@ export function runMiddleware(
 const moduleCache = new Map<string, Promise<Record<string, unknown>>>();
 const moduleVersions = new Map<string, number>();
 const browserArtifactPaths = new Map<string, string>();
+const islandArtifactPaths = new Map<string, string>();
 
 type ImportMode = "legacy" | "compatible" | "explicit";
 interface CompileImportOptions {
@@ -644,6 +645,25 @@ export function serveWrnBrowserArtifact(pathname: string): Response | null {
   });
 }
 
+/** Registers a built island asset for serving under `/__wrnexus/island/`. */
+export function registerIslandArtifact(pathname: string, artifact: string): void {
+  islandArtifactPaths.set(pathname, artifact);
+}
+
+/** Serves a built island bundle, chunk, or the island mount runtime. */
+export function serveIslandArtifact(pathname: string): Response | null {
+  const artifact = islandArtifactPaths.get(pathname);
+  if (!artifact || !existsSync(artifact)) return null;
+  return new Response(readFileSync(artifact, "utf8"), {
+    headers: {
+      "content-type": "text/javascript; charset=utf-8",
+      "cache-control": "no-store, max-age=0",
+      pragma: "no-cache",
+      expires: "0",
+    },
+  });
+}
+
 /** Forget one module and force its next dynamic import to bypass Bun's import cache. */
 export function invalidateModule(file: string): void {
   file = resolve(file);