test(benchmarks): add A/B task fixtures with hidden checks

Ten small Python tasks, each failing on the fixture and passing with a
reference solution, including workflow expectations for a small change, a
bugfix, and a delegable change.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

scripts/benchmarks/tasks/10-performance-dedupe/hidden/test_hidden.py

@@ -0,0 +1,16 @@
+import time
+import unittest
+from app.dedupe import dedupe
+
+
+class Hidden(unittest.TestCase):
+    def test_behavior(self):
+        self.assertEqual(dedupe([3, 1, 3, 2, 1]), [3, 1, 2])
+        self.assertEqual(dedupe([]), [])
+        self.assertEqual(dedupe(['a', 'b', 'a']), ['a', 'b'])
+
+    def test_fast(self):
+        items = list(range(100000)) * 2
+        start = time.perf_counter()
+        self.assertEqual(len(dedupe(items)), 100000)
+        self.assertLess(time.perf_counter() - start, 1.0)