test(cp5): guard viz package against eager matplotlib import

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

tests/viz/test_import_safety.py

@@ -0,0 +1,17 @@
+import sys
+
+
+def test_importing_viz_does_not_import_matplotlib():
+    # Drop any pre-loaded matplotlib so we observe a fresh import graph.
+    for name in list(sys.modules):
+        if name == "matplotlib" or name.startswith("matplotlib."):
+            del sys.modules[name]
+    for name in list(sys.modules):
+        if name == "proteus.viz" or name.startswith("proteus.viz."):
+            del sys.modules[name]
+
+    import proteus.viz  # noqa: F401
+
+    assert "matplotlib" not in sys.modules, (
+        "importing proteus.viz pulled in matplotlib; keep it lazy in write_pngs"
+    )