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/cli/src/build.ts

@@ -24,6 +24,7 @@ import {
 import { basename, dirname, extname, join, relative, resolve } from "node:path";
 import { buildRouter, type Route } from "@wrnexus/router";
 import { getComponentControllerRuntime, getReactiveRuntime } from "@wrnexus/csr";
+import { getIslandRuntime } from "@wrnexus/react/runtime";
 import {
   analyzeRuntimeImports,
   analyzeRuntimeRequirements,
@@ -112,6 +113,7 @@ export async function runBuild(appRoot: string): Promise<void> {
   const clientModulesDir = join(distDir, "client");
   const reactivePath = join(distDir, "reactive.js");
   const controllersPath = join(distDir, "controllers.js");
+  const islandsPath = join(distDir, "islands.js");
   const publicDir = join(root, "public");
   const distPublicDir = join(distDir, "public");
   const config = await loadAppConfig(root);
@@ -493,6 +495,16 @@ export async function runBuild(appRoot: string): Promise<void> {
   assetHash.update(controllerCode);
   console.log(`✓ Controllers: ${controllersPath}`);
 
+  // Island bootstrap: emitted unconditionally but inert without markers, so a
+  // build with no islands still ships no React.
+  const islandCode = await buildBrowserRuntime(
+    getIslandRuntime(),
+    islandsPath,
+    join(compiledDir, "islands.entry.js"),
+  );
+  assetHash.update(islandCode);
+  console.log(`✓ Islands: ${islandsPath}`);
+
   // 1a) Theme tokens + client switcher (always emitted; built-in light/dark).
   const theme = resolveThemeConfig(config.theme, config.cookies);
   const themeCss = renderThemeCss(theme);