Add software-rendered Cube Runner demo

[wysaid]

Summary

• add a self-contained Cube Runner game under demo/
• render a curved 12-lane tunnel, obstacles, player, lighting, fog, and HUD entirely on the CPU
• include eight obstacle/stage patterns, collision handling, scoring, lives, pause, and restart controls
• keep the demo asset-free and compact enough for a 640 x 480 window
• include a headless build mode so the same renderer can produce deterministic validation frames

Motivation

This demonstrates how XEGE can host a small 3D game without requiring a hardware-accelerated backend. The software rasterizer uses filled triangles, a depth buffer, fog, and lightweight face shading, making it useful both as a playable demo and as a reference for CPU-rendered 3D effects.

Controls

• A / D or arrow keys: steer around the tunnel
• P: pause or resume
• R: restart
• Esc: exit

Validation

• MinGW Release build of game_cube_runner
• MinGW Release build of the complete demos target
• MinGW Release default all-target build
• native headless build with -Wall -Wextra -Wpedantic
• headless runtime validation with AddressSanitizer and UndefinedBehaviorSanitizer
• deterministic 640 x 480 render inspected visually

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch demo/cube-runner

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