test(helpers): add the shared temp-repo lifecycle helper

`createTempDirPool` gives a suite one owner for its temp fixture repos —
create on demand, remove them all in one `afterAll` — instead of a hand-rolled
mkdtemp/rmSync pair per file. The PDG receiver pin added in the next commit
uses it.

Cherry-picked verbatim from ec36c6dda on the #2802 branch, where it was
extracted to collapse five hand-rolled cleanups. Identical content, so if both
branches land the add resolves as a duplicate rather than a divergence.

Refs #2807

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

gitnexus/test/helpers/temp-dir-pool.ts

@@ -0,0 +1,62 @@
+/**
+ * Temp-directory lifecycle for pipeline-level integration tests.
+ *
+ * The pipeline mutates the repo it is handed (parse caches, `.gitnexus/`), so
+ * these tests each run against a throwaway copy of a fixture. Every consumer
+ * had hand-rolled the SAME three parts — a `string[]` of created dirs, a
+ * `mkdtempSync` that pushes onto it, and an `afterAll` that `rmSync`s the lot.
+ * Extracted at the fourth consumer (`pipeline-pdg`, `pipeline-pdg-streaming`,
+ * `interproc-taint`, `pdg-chained-receiver-callees`); the copies had already
+ * drifted — `pipeline-pdg` registered two cleanup hooks over one array.
+ *
+ * Only the LIFECYCLE is shared, deliberately: seeding differs per test (a
+ * recursive fixture copy, a single file, an inline-written source, or nothing
+ * at all), so `dir()` hands back an empty registered directory and the caller
+ * fills it however it likes. `fromFixture()` is the common case.
+ *
+ * `createTempDirPool` calls `afterAll` itself, so it must be called from a
+ * test file's module scope (not from this module's top level — ESM caching
+ * would register the hook once, for whichever file imported it first).
+ * Directories are registered at creation, before any seeding runs, so a
+ * fixture copy or a pipeline run that throws still leaves them cleaned up.
+ */
+
+import fs from 'fs';
+import os from 'os';
+import path from 'path';
+import { afterAll } from 'vitest';
+
+export interface TempDirPool {
+  /** A fresh empty temp dir, registered for cleanup. Seed it yourself. */
+  dir(): string;
+  /** A fresh temp dir seeded with a recursive copy of `fixture`. */
+  fromFixture(fixture: string): string;
+}
+
+/**
+ * Create a pool of temp directories that are removed after the calling test
+ * file finishes. `prefix` is the `mkdtemp` prefix (e.g. `'gn-pdg-'`), kept
+ * per-pool so a leaked directory still names the suite that made it.
+ */
+export function createTempDirPool(prefix: string): TempDirPool {
+  const created: string[] = [];
+
+  const dir = (): string => {
+    const made = fs.mkdtempSync(path.join(os.tmpdir(), prefix));
+    created.push(made);
+    return made;
+  };
+
+  afterAll(() => {
+    for (const d of created) fs.rmSync(d, { recursive: true, force: true });
+  });
+
+  return {
+    dir,
+    fromFixture(fixture: string): string {
+      const made = dir();
+      fs.cpSync(fixture, made, { recursive: true });
+      return made;
+    },
+  };
+}