M1 Grid and movement. GameState per SPEC.md, step() advances the head, tail vacates, reversal ignored. M2 Food and growth. Seeded spawn, eating increments score and pending_growth, food never spawns on the body. M3 Collision and game over. Walls and self, tail-vacate rule per spec. M4 Determinism and speed hook. Same seed produces identical food sequence; engine exposes score for the renderer's tick rate. M5 Pygame renderer (snake/render.py). Reads engine state, draws, maps arrow keys to Direction, tick rate ramps every 5 foods. Human-verified, no automated tests.