chore: import upstream snapshot with attribution

tests/unit/isolate-datadir.test.ts

@@ -0,0 +1,45 @@
+// Regression guard for tests/_setup/isolateDataDir.ts — the test-only module that
+// gives each test process its own DATA_DIR so concurrent test files never share the
+// on-disk SQLite DB. Removing or breaking it brings back the cross-file state races
+// (the `test:unit` hang under high concurrency and the non-deterministic Stryker
+// baseline that forced concurrency: 1).
+import test from "node:test";
+import assert from "node:assert/strict";
+import { spawnSync } from "node:child_process";
+import os from "node:os";
+
+function dataDirFromChild(envDataDir: string | undefined): string {
+  const result = spawnSync(
+    process.execPath,
+    [
+      "--import",
+      "tsx",
+      "--import",
+      "./tests/_setup/isolateDataDir.ts",
+      "-e",
+      "console.log(process.env.DATA_DIR ?? '')",
+    ],
+    {
+      encoding: "utf8",
+      cwd: process.cwd(),
+      // Pass DATA_DIR through verbatim; an empty string means "unset" for the module's
+      // `if (!process.env.DATA_DIR)` guard.
+      env: { ...process.env, DATA_DIR: envDataDir ?? "" },
+    }
+  );
+  return result.stdout.trim().split("\n").pop() ?? "";
+}
+
+test("isolateDataDir assigns a unique temp DATA_DIR when none is set", () => {
+  const a = dataDirFromChild(undefined);
+  const b = dataDirFromChild(undefined);
+
+  assert.ok(a.startsWith(os.tmpdir()), `expected a temp dir under ${os.tmpdir()}, got ${a}`);
+  assert.match(a, /omniroute-test-/, `expected the omniroute-test- prefix, got ${a}`);
+  assert.notEqual(a, b, "two processes must each get their own DATA_DIR");
+});
+
+test("isolateDataDir respects an explicitly set DATA_DIR", () => {
+  const explicit = "/tmp/omniroute-explicit-fixture";
+  assert.equal(dataDirFromChild(explicit), explicit);
+});