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/PyWindow.cpp

@@ -2,6 +2,7 @@
 #include "GameEngine.h"
 #include "McRFPy_API.h"
 #include <SFML/Graphics.hpp>
+#include <cstring>
 
 // Singleton instance - static variable, not a class member
 static PyWindowObject* window_instance = nullptr;
@@ -404,6 +405,82 @@ PyObject* PyWindow::screenshot(PyWindowObject* self, PyObject* args, PyObject* k
     return PyBytes_FromStringAndSize((const char*)pixels, size.x * size.y * 4);
 }
 
+PyObject* PyWindow::get_game_resolution(PyWindowObject* self, void* closure)
+{
+    GameEngine* game = McRFPy_API::game;
+    if (!game) {
+        PyErr_SetString(PyExc_RuntimeError, "No game engine initialized");
+        return NULL;
+    }
+    
+    auto resolution = game->getGameResolution();
+    return Py_BuildValue("(ii)", resolution.x, resolution.y);
+}
+
+int PyWindow::set_game_resolution(PyWindowObject* self, PyObject* value, void* closure)
+{
+    GameEngine* game = McRFPy_API::game;
+    if (!game) {
+        PyErr_SetString(PyExc_RuntimeError, "No game engine initialized");
+        return -1;
+    }
+    
+    int width, height;
+    if (!PyArg_ParseTuple(value, "ii", &width, &height)) {
+        PyErr_SetString(PyExc_TypeError, "game_resolution must be a tuple of two integers (width, height)");
+        return -1;
+    }
+    
+    if (width <= 0 || height <= 0) {
+        PyErr_SetString(PyExc_ValueError, "Game resolution dimensions must be positive");
+        return -1;
+    }
+    
+    game->setGameResolution(width, height);
+    return 0;
+}
+
+PyObject* PyWindow::get_scaling_mode(PyWindowObject* self, void* closure)
+{
+    GameEngine* game = McRFPy_API::game;
+    if (!game) {
+        PyErr_SetString(PyExc_RuntimeError, "No game engine initialized");
+        return NULL;
+    }
+    
+    return PyUnicode_FromString(game->getViewportModeString().c_str());
+}
+
+int PyWindow::set_scaling_mode(PyWindowObject* self, PyObject* value, void* closure)
+{
+    GameEngine* game = McRFPy_API::game;
+    if (!game) {
+        PyErr_SetString(PyExc_RuntimeError, "No game engine initialized");
+        return -1;
+    }
+    
+    const char* mode_str = PyUnicode_AsUTF8(value);
+    if (!mode_str) {
+        PyErr_SetString(PyExc_TypeError, "scaling_mode must be a string");
+        return -1;
+    }
+    
+    GameEngine::ViewportMode mode;
+    if (strcmp(mode_str, "center") == 0) {
+        mode = GameEngine::ViewportMode::Center;
+    } else if (strcmp(mode_str, "stretch") == 0) {
+        mode = GameEngine::ViewportMode::Stretch;
+    } else if (strcmp(mode_str, "fit") == 0) {
+        mode = GameEngine::ViewportMode::Fit;
+    } else {
+        PyErr_SetString(PyExc_ValueError, "scaling_mode must be 'center', 'stretch', or 'fit'");
+        return -1;
+    }
+    
+    game->setViewportMode(mode);
+    return 0;
+}
+
 // Property definitions
 PyGetSetDef PyWindow::getsetters[] = {
     {"resolution", (getter)get_resolution, (setter)set_resolution, 
@@ -418,6 +495,10 @@ PyGetSetDef PyWindow::getsetters[] = {
      "Window visibility state", NULL},
     {"framerate_limit", (getter)get_framerate_limit, (setter)set_framerate_limit,
      "Frame rate limit (0 for unlimited)", NULL},
+    {"game_resolution", (getter)get_game_resolution, (setter)set_game_resolution,
+     "Fixed game resolution as (width, height) tuple", NULL},
+    {"scaling_mode", (getter)get_scaling_mode, (setter)set_scaling_mode,
+     "Viewport scaling mode: 'center', 'stretch', or 'fit'", NULL},
     {NULL}
 };