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_module_simple.py

@@ -0,0 +1,22 @@
+#!/usr/bin/env python3
+"""Simple module test"""
+import sys
+import mcrfpy
+
+print("Step 1: Module loaded")
+
+# Test window singleton exists (#184)
+print("Step 2: Checking window...")
+has_window = hasattr(mcrfpy, 'window')
+print(f"  has window: {has_window}")
+
+if has_window:
+    print("Step 3: Getting window...")
+    window = mcrfpy.window
+    print(f"  window: {window}")
+    print("Step 4: Checking resolution...")
+    res = window.resolution
+    print(f"  resolution: {res}")
+
+print("PASS")
+sys.exit(0)