Stabilize dev single-build error paths

Guard stats access and reject when finishOutput fails in the dev single-build path. Prevents silent hangs and ensures CI/dev runs surface errors promptly

Author: PeterDaveHello

build.mjs

@@ -562,13 +562,17 @@ async function build() {
 
   await new Promise((resolve, reject) =>
     runWebpack(false, false, false, outdir, async (err, stats) => {
-      if (err || stats.hasErrors()) {
+      if (err || (stats && typeof stats.hasErrors === 'function' && stats.hasErrors())) {
         console.error(err || stats.toString())
         reject(err || new Error('webpack error'))
         return
       }
-      await finishOutput('')
-      resolve()
+      try {
+        await finishOutput('')
+        resolve()
+      } catch (e) {
+        reject(e)
+      }
     }),
   )
 }