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/test_scene_create.py

@@ -0,0 +1,7 @@
+#!/usr/bin/env python3
+import sys
+import mcrfpy
+print("Creating scene...")
+scene = mcrfpy.Scene("test")
+print("Scene created")
+sys.exit(0)