feat(runtime): implement esbuild-wasm transpiler with lazy initialization

- Load esbuild-wasm from CDN (pinned v0.20.2) — zero npm dependencies
- Implement Promise-based lazy init guard for concurrent call safety
- Expose transpile() with ESM output, esnext target, inline sourcemaps
- Disable nested worker (worker: false) for ServiceWorker compat

Author: singhjashanjot

packages/runtime/src/transpiler.ts

@@ -0,0 +1,25 @@
+import * as esbuild from 'https://unpkg.com/esbuild-wasm@0.20.2/esm/browser.js'
+
+let initPromise: Promise<void> | null = null
+
+async function ensureInitialized(): Promise<void> {
+  if (!initPromise) {
+    initPromise = esbuild.initialize({
+      wasmURL: 'https://unpkg.com/esbuild-wasm@0.20.2/esbuild.wasm',
+      worker: false,
+    })
+  }
+  await initPromise
+}
+
+export async function transpile(source: string, filename: string): Promise<string> {
+  await ensureInitialized()
+  const result = await esbuild.transform(source, {
+    loader: 'ts',
+    format: 'esm',
+    target: 'esnext',
+    sourcefile: filename,
+    sourcemap: 'inline',
+  })
+  return result.code
+}