Organize test suite: add README, move loose tests to proper directories

- Add tests/README.md documenting test structure and usage
- Move issue_*_test.py files to tests/regression/ (9 files)
- Move loose test_*.py files to tests/unit/ (18 files)
- tests/ root now contains only pytest infrastructure

Addresses #166

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/unit/minimal_reparent.py

@@ -0,0 +1,14 @@
+import mcrfpy
+scene = mcrfpy.Scene("test")
+scene.activate()
+f1 = mcrfpy.Frame((10,10), (100,100), fill_color = (255, 0, 0, 64))
+f2 = mcrfpy.Frame((200,10), (100,100), fill_color = (0, 255, 0, 64))
+f_child = mcrfpy.Frame((25,25), (50,50), fill_color = (0, 0, 255, 64))
+
+scene.children.append(f1)
+scene.children.append(f2)
+f1.children.append(f_child)
+f_child.parent = f2
+
+print(f1.children)
+print(f2.children)