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_gridpoint_debug.py

@@ -0,0 +1,15 @@
+#!/usr/bin/env python3
+import sys
+import mcrfpy
+print("1 - Loading texture", flush=True)
+texture = mcrfpy.Texture("assets/kenney_ice.png", 16, 16)
+print("2 - Creating grid", flush=True)
+grid = mcrfpy.Grid(grid_size=(10, 10), texture=texture, pos=(0, 0), size=(160, 160))
+print("3 - Getting grid point at (3, 5)", flush=True)
+point = grid.at(3, 5)
+print(f"4 - Point: {point}", flush=True)
+print("5 - Getting grid_pos", flush=True)
+grid_pos = point.grid_pos
+print(f"6 - grid_pos: {grid_pos}", flush=True)
+print("PASS", flush=True)
+sys.exit(0)