Harden CPU count and cache key; set exit code on compiler close errors

Safely detect CPU count with try/catch fallback to avoid init crashes. Use a filesystem-safe cache key by joining parts with '__'. Mark
process exit code when compiler.close reports an error in prod and ensure unhandled build failures exit non-zero.

Author: PeterDaveHello

build.mjs

@@ -32,7 +32,14 @@ if (cacheCompressionEnv != null) {
     cacheCompressionOption = false
   }
 }
-const cpuCount = os.cpus().length || 1
+let cpuCount = 1
+try {
+  const cpuInfo = os.cpus && os.cpus()
+  const len = Array.isArray(cpuInfo) ? cpuInfo.length : 0
+  cpuCount = Number.isInteger(len) && len > 0 ? len : 1
+} catch {
+  cpuCount = 1
+}
 const rawWorkers = process.env.BUILD_THREAD_WORKERS
   ? parseInt(process.env.BUILD_THREAD_WORKERS, 10)
   : undefined
@@ -97,13 +104,14 @@ async function runWebpack(isWithoutKatex, isWithoutTiktoken, minimal, sourceBuil
   }
 
   const dirKey = path.basename(sourceBuildDir || outdir)
-  const variantId = [
+  const variantParts = [
     isWithoutKatex ? 'no-katex' : 'with-katex',
     isWithoutTiktoken ? 'no-tiktoken' : 'with-tiktoken',
     minimal ? 'minimal' : 'full',
     dirKey,
     isProduction ? 'prod' : 'dev',
-  ].join('|')
+  ]
+  const variantId = variantParts.join('__')
 
   const compiler = webpack({
     entry: {
@@ -373,7 +381,10 @@ async function runWebpack(isWithoutKatex, isWithoutTiktoken, minimal, sourceBuil
     compiler.run((err, stats) => {
       const finishClose = () =>
         compiler.close((closeErr) => {
-          if (closeErr) console.error('Error closing compiler:', closeErr)
+          if (closeErr) {
+            console.error('Error closing compiler:', closeErr)
+            process.exitCode = 1
+          }
         })
       try {
         const ret = callback(err, stats)
@@ -581,4 +592,7 @@ async function build() {
   )
 }
 
-build()
+build().catch((e) => {
+  console.error(e)
+  process.exit(1)
+})