feat(islands): wire islands end to end

The island pieces existed but nothing connected .wrn compilation to island
emission. Now:

- codegen emits a data-wrn-island placeholder for component tags bound to
  .tsx imports, keeping .wrn components on the normal mount path
- the dev pipeline and static build resolve island imports, thread the
  names into codegen, and build the bundles
- collectScripts adds /__wrnexus/islands.js only when island markup is
  present, so island-free pages still ship nothing
- island routes classify as static-interactive via hasIslands

Three bugs found by driving a real page in the browser:

1. The mount runtime was never built anywhere, so the bootstrap 404'd and
   no island mounted.
2. Building the runtime separately from the islands gave each its own copy
   of React: "Cannot read properties of null (reading 'useState')". The
   runtime is now an entrypoint of the same build so React stays in one
   shared chunk. The existing single-React test only compared bundles
   within one build and could not see across build boundaries.
3. Island props arrived as attribute strings, so start={3} was "3" and
   incrementing produced "31" then "311". Props now follow JSX semantics:
   {…} parses as JSON, quoted values stay strings, and a runtime
   expression is a WRN-ISLAND-PROPS build error rather than a silent
   wrong value.

island-codegen.ts no longer imports @wrnexus/core. Compiler modules are
bundled into the Node-only VS Code extension, which contains no other
packages, so a runtime import of core broke the editor compiler; the two
helpers are implemented locally and the core dependency is dropped again.

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

packages/compiler/src/island-bundle.ts

@@ -82,6 +82,16 @@ export function assertReactAvailable(
 export async function buildIslands(input: {
   islands: IslandInput[];
   outDir: string;
+  /**
+   * App root used to resolve the island mount runtime. When given, the runtime
+   * is emitted as `runtime.js` in the SAME build as the islands.
+   *
+   * This is not a convenience: building the runtime separately gives it its own
+   * copy of React, and a component rendered by one copy while importing hooks
+   * from another fails with "Cannot read properties of null (reading
+   * 'useState')". One build with splitting keeps React in a single shared chunk.
+   */
+  appRoot?: string;
 }): Promise<IslandBuildResult> {
   if (input.islands.length === 0) return { assets: [], sharedChunks: [] };
 
@@ -100,9 +110,22 @@ export async function buildIslands(input: {
     writeFileSync(join(entryDir, `${island.name}.tsx`), generateIslandEntry(island), "utf8");
   }
 
+  const entrypoints = input.islands.map((island) => join(entryDir, `${island.name}.tsx`));
+  if (input.appRoot) {
+    const resolveFrom = createRequire(join(input.appRoot, "package.json"));
+    const runtimeEntry = join(entryDir, "runtime.ts");
+    writeFileSync(
+      runtimeEntry,
+      `export * from ${JSON.stringify(resolveFrom.resolve("@wrnexus/react/browser"))};
+`,
+      "utf8",
+    );
+    entrypoints.push(runtimeEntry);
+  }
+
   try {
     const result = await Bun.build({
-      entrypoints: input.islands.map((island) => join(entryDir, `${island.name}.tsx`)),
+      entrypoints,
       outdir: input.outDir,
       target: "browser",
       format: "esm",
@@ -123,6 +146,7 @@ export async function buildIslands(input: {
         // Bun names an entry's output after its entry file, so the basename
         // identifies the island unambiguously.
         const stem = basename(output.path).replace(/\.js$/, "");
+        if (stem === "runtime") continue;
         if (!islandNames.has(stem)) continue;
         assets.push({
           name: stem,