build: isolated dev store (.next-dev) — a prod build can no longer kill a live dev server

.gitignore

@@ -11,8 +11,9 @@ node_modules/
 /coverage
 /test-results/
 
-# next.js
+# next.js (dev and prod build stores — isolated, see next.config.js)
 /.next/
+/.next-dev/
 /out/
 
 # production

next.config.js

@@ -6,6 +6,10 @@ module.exports = {
   // nginx. Every route is prerendered at build; ISR is replaced by
   // rebuild-on-push.
   output: 'export',
+  // Isolated build stores: dev writes `.next-dev`, prod writes `.next`. A
+  // `yarn build` under a live dev server used to overwrite `.next` under it —
+  // chunk paths went stale and the dev runtime died mid-request. Never again.
+  distDir: process.env.NODE_ENV === 'development' ? '.next-dev' : '.next',
   images: {
     // `next/image` optimization needs a server; flat files ship originals.
     unoptimized: true,