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/test_iteration.py

@@ -1,43 +0,0 @@
-#!/usr/bin/env python3
-"""Test iteration works with hidden types"""
-import sys
-import mcrfpy
-
-print("Step 1: Creating scene...")
-scene = mcrfpy.Scene("test_scene")
-print(f"  scene: {scene}")
-
-print("Step 2: Getting children...")
-ui = scene.children
-print(f"  children: {ui}")
-
-print("Step 3: Creating Frame...")
-frame = mcrfpy.Frame(pos=(0,0), size=(50,50))
-print(f"  frame: {frame}")
-
-print("Step 4: Appending Frame...")
-ui.append(frame)
-print(f"  append succeeded, len={len(ui)}")
-
-print("Step 5: Creating Caption...")
-caption = mcrfpy.Caption(text="hi", pos=(0,0))
-print(f"  caption: {caption}")
-
-print("Step 6: Appending Caption...")
-ui.append(caption)
-print(f"  append succeeded, len={len(ui)}")
-
-print("Step 7: Starting iteration...")
-count = 0
-for item in ui:
-    count += 1
-    print(f"  Item {count}: {item}")
-
-print(f"Step 8: Iteration complete, {count} items")
-
-if count == 2:
-    print("PASS")
-    sys.exit(0)
-else:
-    print(f"FAIL: expected 2 items, got {count}")
-    sys.exit(1)