feat(learner): JS/TS exercise runner in a Web Worker with timeout and network blocked

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/feature-learner/src/server/transpile.ts

@@ -0,0 +1,27 @@
+import type { Feature } from "@stc/contracts";
+import { badRequest, body, json } from "./http";
+
+const MAX = 200_000;
+const ts = new Bun.Transpiler({ loader: "ts", target: "browser" });
+
+/** Type-strips TypeScript for the in-browser runner (the worker itself runs plain JavaScript). */
+export function transpileTs(code: string): string {
+  return ts.transformSync(code);
+}
+
+export function transpileRoutes(): Feature["routes"] {
+  return {
+    "/api/learner/transpile": {
+      POST: async (req) => {
+        const b = await body<{ code?: string }>(req);
+        if (!b || typeof b.code !== "string") return badRequest("code is required");
+        if (b.code.length > MAX) return badRequest("code is too long");
+        try {
+          return json({ code: transpileTs(b.code) });
+        } catch (e) {
+          return json({ error: (e as Error).message }, 422);
+        }
+      },
+    },
+  };
+}