HeightMap: add GRID_MAX limit and input validation

Fixes potential integer overflow and invalid input issues:

- Add GRID_MAX constant (8192) to Common.h for global use
- Validate HeightMap dimensions against GRID_MAX to prevent
  integer overflow in w*h calculations (65536*65536 = 0)
- Add min > max validation for clamp() and normalize()
- Add unit tests for all new validation cases

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

src/Common.h

@@ -2,6 +2,10 @@
 #include <SFML/Graphics.hpp>
 #include <SFML/Audio.hpp>
 
+// Maximum dimension for grids, layers, and heightmaps (8192x8192 = 256MB of float data)
+// Prevents integer overflow in size calculations and limits memory allocation
+constexpr int GRID_MAX = 8192;
+
 #include <vector>
 #include <iostream>
 #include <memory>