feat: Verbatim MVP - terminal renderer, cellular automaton, Verlet physics

- World: 250x250 grid with 14 materials (sand, water, lava, stone, wood, etc.)
- Physics: cellular automaton for materials + Verlet solver for entities
- Entity: multi-cell humanoid (7 sub-bodies with distance constraints)
- Render: terminal renderer with ANSI colors and diff-based updates
- Game loop: fixed 60Hz timestep with accumulator pattern
- Input: WASD movement, number keys for material painting

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