test: add regression test for import time (<3s)

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

Author: leotrs

tests/test_import_time.py

@@ -0,0 +1,16 @@
+import subprocess
+import sys
+
+
+def test_import_time():
+    """import xgi should complete in under 3 seconds."""
+    result = subprocess.run(
+        [sys.executable, "-X", "importtime", "-c", "import xgi"],
+        capture_output=True,
+        text=True,
+    )
+    # last line of importtime stderr: "import time: self | cumulative | xgi"
+    last_line = result.stderr.strip().splitlines()[-1]
+    cumulative_us = int(last_line.split("|")[1].strip())
+    cumulative_s = cumulative_us / 1_000_000
+    assert cumulative_s < 3, f"import xgi took {cumulative_s:.2f}s, expected < 3s"