CRITICAL white-screen bug from the build-output-isolation refactor: the standalone
server.js bakes distDir ("./.build/next") into its config and serves /_next/static
from <root>/.build/next/static — but assembleStandalone hard-coded the destination
to <outDir>/.next/static (+ sanitised/patched <outDir>/.next/{required-server-files,
server}). Result: the server's static dir was EMPTY → every JS/CSS chunk 404'd →
blank login page (health stayed 200, so it slipped past the health-only dry-run).
Mirror the distDir path (relative to projectRoot) for static, required-server-files
sanitization (was a silent no-op → 0 paths sanitised, now 11), and the Turbopack
chunk patch. Verified: booting the assembled bundle serves the webpack chunk 200.
Affects every consumer (npm/Docker/Electron/VPS).
Extract the shared copy/sync/sanitize logic (native assets, extra modules,
static/public, optional path-sanitize + turbopack-chunk-patch) from the three
divergent assembly scripts into one module. Wire build-next-isolated.mjs to call
it (in-place .next/standalone). Output is byte-identical to before — pure refactor.
Golden test + existing build-next-isolated tests (7/7) green; standalone retains
all natives + sidecars.
