feat(config): add build-time turbopack.root validation

Add programmatic assertion in next.config.ts that validates turbopack.root
is an absolute path pointing to an existing directory. Throws at build time
if contract is violated.

Update implementation-guide.md to reference the automated validation.

Author: BjornMelin

docs/review/2025-12-15/implementation-guide.md

@@ -122,9 +122,9 @@ Goal: restore “merge-safe” invariants (build passes, CI enforces build, base
     - `next.config.ts`
   - Steps:
     - Delete `turbopack.root` entirely (preferred) unless you actually depend on linked/workspace resolution outside repo root, or Next is inferring the root from an unrelated lockfile in a parent directory.
-    - If you keep it, set it to an absolute path. Add a build-time assertion in `next.config.ts` that validates `turbopack.root` is an absolute path pointing to an existing directory (throw on failure).
+    - If you keep it, set it to an absolute path. The build-time assertion in `next.config.ts` validates that `turbopack.root` is absolute and points to an existing directory (throws on failure).
   - Verify:
-    - `pnpm build` emits no Turbopack root warnings and the programmatic assertion passes.
+    - `pnpm build` succeeds; the programmatic assertion in `next.config.ts` enforces the absolute-path contract automatically.
   - References:
     - <https://nextjs.org/docs/pages/api-reference/config/next-config-js/turbopack>
 

next.config.ts

@@ -2,14 +2,23 @@
  * @fileoverview Next.js configuration for TripSage AI (Turbopack root, build optimizations, and security headers).
  */
 
-import { dirname } from "node:path";
+import { existsSync } from "node:fs";
+import { dirname, isAbsolute } from "node:path";
 import { fileURLToPath } from "node:url";
 import withBundleAnalyzer from "@next/bundle-analyzer";
 import { withBotId } from "botid/next/config";
 import type { NextConfig } from "next";
 
 const TURBOPACK_ROOT = dirname(fileURLToPath(import.meta.url));
 
+// Build-time assertion: turbopack.root must be absolute and exist
+if (!isAbsolute(TURBOPACK_ROOT)) {
+  throw new Error(`turbopack.root must be an absolute path, got: ${TURBOPACK_ROOT}`);
+}
+if (!existsSync(TURBOPACK_ROOT)) {
+  throw new Error(`turbopack.root must point to an existing directory: ${TURBOPACK_ROOT}`);
+}
+
 const nextConfig: NextConfig = {
   // Enable Cache Components (required for "use cache" directives in codebase)
   cacheComponents: true,