feat(viewport): complete viewport-based rendering system (#8)

Implements a comprehensive viewport system that allows fixed game resolution
with flexible window scaling, addressing the primary wishes for issues #34, #49, and #8.

Key Features:
- Fixed game resolution independent of window size (window.game_resolution property)
- Three scaling modes accessible via window.scaling_mode:
  - "center": 1:1 pixels, viewport centered in window
  - "stretch": viewport fills window, ignores aspect ratio
  - "fit": maintains aspect ratio with black bars
- Automatic window-to-game coordinate transformation for mouse input
- Full Python API integration with PyWindow properties

Technical Implementation:
- GameEngine::ViewportMode enum with Center, Stretch, Fit modes
- SFML View system for efficient GPU-based viewport scaling
- updateViewport() recalculates on window resize or mode change
- windowToGameCoords() transforms mouse coordinates correctly
- PyScene mouse input automatically uses transformed coordinates

Tests:
- test_viewport_simple.py: Basic API functionality
- test_viewport_visual.py: Visual verification with screenshots
- test_viewport_scaling.py: Interactive mode switching and resizing

This completes the viewport-based rendering task and provides the foundation
for resolution-independent game development as requested for Crypt of Sokoban.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

src/PyScene.cpp

@@ -28,7 +28,8 @@ void PyScene::do_mouse_input(std::string button, std::string type)
     }
     
     auto unscaledmousepos = sf::Mouse::getPosition(game->getWindow());
-    auto mousepos = game->getWindow().mapPixelToCoords(unscaledmousepos);
+    // Convert window coordinates to game coordinates using the viewport
+    auto mousepos = game->windowToGameCoords(sf::Vector2f(unscaledmousepos));
     
     // Create a sorted copy by z-index (highest first)
     std::vector<std::shared_ptr<UIDrawable>> sorted_elements(*ui_elements);