feat(generators): core-package-utils (assertNotPresent + addToTranspilePackages)

Two helpers for use by per-package generator actions:
- assertOptionalPackageNotPresent — guards against double-scaffolding
- addToTranspilePackages — idempotent alphabetical splice into next.config.mjs

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

turbo/generators/lib/core-package-utils.ts

@@ -0,0 +1,44 @@
+import { existsSync, readFileSync, writeFileSync } from "node:fs";
+import { join } from "node:path";
+
+/**
+ * Throws if a core package directory already exists. Used as the first action
+ * in every per-package generator so re-running is safe.
+ */
+export function assertOptionalPackageNotPresent(
+  name: string,
+  cwd: string = process.cwd(),
+): void {
+  const pkgRoot = join(cwd, "packages", name);
+  if (existsSync(pkgRoot)) {
+    throw new Error(
+      `packages/${name}/ already exists — refusing to scaffold (delete it first if intentional)`,
+    );
+  }
+}
+
+/**
+ * Inserts a package name into the transpilePackages array of a Next.js config
+ * file, preserving alphabetical order. Idempotent.
+ */
+export function addToTranspilePackages(
+  nextConfigPath: string,
+  pkgName: string,
+): void {
+  const source = readFileSync(nextConfigPath, "utf8");
+  if (source.includes(`"${pkgName}"`)) return;
+  const updated = source.replace(
+    /(transpilePackages:\s*\[\s*)([\s\S]*?)(\s*\])/,
+    (_match, open: string, body: string, close: string) => {
+      const entries = body
+        .split(",")
+        .map((e) => e.trim())
+        .filter(Boolean);
+      entries.push(`"${pkgName}"`);
+      entries.sort();
+      const formatted = entries.map((e) => `    ${e}`).join(",\n");
+      return `${open}\n${formatted},${close}`;
+    },
+  );
+  writeFileSync(nextConfigPath, updated);
+}