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>

ROADMAP.md

@@ -65,6 +65,7 @@
   - Grid background colors (#50) ✅
   - RenderTexture base infrastructure ✅
   - UIFrame clipping support ✅
+  - Viewport-based rendering (#8) ✅
 
 ### Active Development:
 - **Branch**: alpha_streamline_2
@@ -244,10 +245,13 @@ Rendering Layer:
    ⏳ Extend to other UI classes
    ⏳ Effects (blur, glow, etc.)
    
-3. #8 - Viewport-based rendering [NEXT PRIORITY]
-   - RenderTexture matches viewport
-   - Proper scaling/letterboxing
-   - Coordinate system transformations
+3. ✅ #8 - Viewport-based rendering [COMPLETED]
+   - Fixed game resolution (window.game_resolution)
+   - Three scaling modes: "center", "stretch", "fit"
+   - Window to game coordinate transformation
+   - Mouse input properly scaled with windowToGameCoords()
+   - Python API fully integrated
+   - Tests: test_viewport_simple.py, test_viewport_visual.py, test_viewport_scaling.py
    
 4. #106 - Shader support [STRETCH GOAL]
    sprite.shader = mcrfpy.Shader.load("glow.frag")
@@ -267,7 +271,8 @@ Rendering Layer:
 - Dirty flag system crucial for performance - only re-render when properties change
 - Nested clipping works correctly with proper coordinate transformations
 - Scene transitions already use RenderTextures - good integration test
-- Next: Viewport rendering (#8) will build on RenderTexture foundation
+- Viewport rendering (#8) ✅ complete with three scaling modes and coordinate transformation
+- Next: Extend RenderTexture support to remaining UI classes (Caption, Sprite, Grid)
 - Shader/Particle systems might be deferred to Phase 7 or Gamma
 
 *Rationale*: This unlocks professional visual effects but is complex.