Consolidate boolean env parsing and cache Sass implementation

- Replace parseBooleanEnv/isDisabledEnv with getBooleanEnv to unify
  boolean env handling while preserving per-flag defaults
- Resolve and cache Sass implementation once per process to avoid
  repeated dynamic imports across build variants

These changes address review suggestions r2331670931 and r2331670979
and keep behavior stable with minimal diff

Author: PeterDaveHello

build.mjs

@@ -15,19 +15,14 @@ const __dirname = path.resolve()
 const isProduction = process.argv[2] !== '--development' // --production and --analyze are both production
 const isAnalyzing = process.argv[2] === '--analyze'
 // Env helpers
-function parseBooleanEnv(val) {
-  if (val == null) return false
+function getBooleanEnv(val, defaultValue) {
+  if (val == null) return defaultValue
   const s = String(val).trim().toLowerCase()
   return !(s === '' || s === '0' || s === 'false' || s === 'no' || s === 'off')
 }
-function isDisabledEnv(val) {
-  if (val == null) return false
-  const s = String(val).trim().toLowerCase()
-  return s === '' || s === '0' || s === 'false' || s === 'no' || s === 'off'
-}
 // Default: parallel build ON unless explicitly disabled
-const parallelBuild = !isDisabledEnv(process.env.BUILD_PARALLEL)
-const isWatchOnce = parseBooleanEnv(process.env.BUILD_WATCH_ONCE)
+const parallelBuild = getBooleanEnv(process.env.BUILD_PARALLEL, true)
+const isWatchOnce = getBooleanEnv(process.env.BUILD_WATCH_ONCE, false)
 // Cache compression control: default none; allow override via env
 function parseCacheCompressionOption(envVal) {
   if (envVal == null) return undefined
@@ -77,7 +72,30 @@ if (process.env.BUILD_POOL_TIMEOUT) {
   }
 }
 // Enable threads by default; allow disabling via BUILD_THREAD=0/false/no/off
-const enableThread = !isDisabledEnv(process.env.BUILD_THREAD)
+const enableThread = getBooleanEnv(process.env.BUILD_THREAD, true)
+
+// Cache and resolve Sass implementation once per process
+let sassImplPromise
+async function getSassImplementation() {
+  if (!sassImplPromise) {
+    sassImplPromise = (async () => {
+      try {
+        const mod = await import('sass-embedded')
+        return mod.default || mod
+      } catch (e1) {
+        try {
+          const mod = await import('sass')
+          return mod.default || mod
+        } catch (e2) {
+          console.error('[build] Failed to load sass-embedded:', e1 && e1.message ? e1.message : e1)
+          console.error('[build] Failed to load sass:', e2 && e2.message ? e2.message : e2)
+          throw new Error("No Sass implementation available. Install 'sass-embedded' or 'sass'.")
+        }
+      }
+    })()
+  }
+  return sassImplPromise
+}
 
 async function deleteOldDir() {
   await fs.rm(outdir, { recursive: true, force: true })
@@ -98,18 +116,7 @@ async function runWebpack(isWithoutKatex, isWithoutTiktoken, minimal, sourceBuil
   ]
   if (isWithoutKatex) shared.push('./src/components')
 
-  let sassImpl
-  try {
-    const mod = await import('sass-embedded')
-    sassImpl = mod.default || mod
-  } catch (e1) {
-    try {
-      const mod = await import('sass')
-      sassImpl = mod.default || mod
-    } catch (e2) {
-      throw new Error("No Sass implementation available. Install 'sass-embedded' or 'sass'.")
-    }
-  }
+  const sassImpl = await getSassImplementation()
 
   const dirKey = path.basename(sourceBuildDir || outdir)
   const variantParts = [