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.test.ts

@@ -0,0 +1,64 @@
+import { describe, it, expect } from "vitest";
+import {
+  existsSync,
+  mkdtempSync,
+  mkdirSync,
+  writeFileSync,
+  readFileSync,
+} from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import {
+  assertOptionalPackageNotPresent,
+  addToTranspilePackages,
+} from "./core-package-utils";
+
+describe("assertOptionalPackageNotPresent", () => {
+  it("throws if packages/<name>/ exists in cwd", () => {
+    const tmp = mkdtempSync(join(tmpdir(), "core-pkg-"));
+    mkdirSync(join(tmp, "packages", "core-foo"), { recursive: true });
+    expect(() => assertOptionalPackageNotPresent("core-foo", tmp)).toThrow(/already exists/);
+  });
+
+  it("returns silently if packages/<name>/ is absent", () => {
+    const tmp = mkdtempSync(join(tmpdir(), "core-pkg-"));
+    expect(() => assertOptionalPackageNotPresent("core-foo", tmp)).not.toThrow();
+  });
+});
+
+describe("addToTranspilePackages", () => {
+  it("inserts package name alphabetically into transpilePackages array", () => {
+    const tmp = mkdtempSync(join(tmpdir(), "core-pkg-"));
+    const cfgPath = join(tmp, "next.config.mjs");
+    writeFileSync(
+      cfgPath,
+      `const nextConfig = {
+  transpilePackages: [
+    "@repo/core-cms",
+    "@repo/core-shared",
+  ],
+};
+`,
+    );
+    addToTranspilePackages(cfgPath, "@repo/core-realtime");
+    const result = readFileSync(cfgPath, "utf8");
+    // Order should be: core-cms, core-realtime, core-shared
+    const order = result.match(/@repo\/core-\w+/g) ?? [];
+    expect(order).toEqual(["@repo/core-cms", "@repo/core-realtime", "@repo/core-shared"]);
+  });
+
+  it("is idempotent — duplicate insertion is a no-op", () => {
+    const tmp = mkdtempSync(join(tmpdir(), "core-pkg-"));
+    const cfgPath = join(tmp, "next.config.mjs");
+    writeFileSync(
+      cfgPath,
+      `const nextConfig = {
+  transpilePackages: ["@repo/core-realtime"],
+};
+`,
+    );
+    addToTranspilePackages(cfgPath, "@repo/core-realtime");
+    const result = readFileSync(cfgPath, "utf8");
+    expect(result.match(/@repo\/core-realtime/g)?.length).toBe(1);
+  });
+});