Initial commit: Multiplayer Snake game with server discovery

Implemented a complete network multiplayer Snake game with the following features:

Core Game:
- Client-server architecture using asyncio for networking
- Pygame-based rendering at 60 FPS
- Server-authoritative game state with 10 TPS
- Collision detection (walls, self, other players)
- Food spawning and score tracking
- Support for multiple players with color-coded snakes

Server Discovery:
- UDP multicast-based automatic server discovery (239.255.0.1:9999)
- Server beacon broadcasts presence every 2 seconds
- Client discovery with 3-second timeout
- Server selection UI for multiple servers
- Auto-connect for single server
- Graceful fallback to manual connection

Project Structure:
- src/shared/ - Protocol, models, constants, discovery utilities
- src/server/ - Game server, game logic, server beacon
- src/client/ - Game client, renderer, discovery, server selector
- tests/ - Unit tests for game logic, models, and discovery

Command-line interface with argparse for both server and client.
Comprehensive documentation in README.md and CLAUDE.md.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/shared/constants.py

@@ -0,0 +1,48 @@
+"""Game constants shared between client and server."""
+
+# Network settings
+DEFAULT_HOST = "localhost"
+DEFAULT_PORT = 8888
+
+# Multicast discovery settings
+MULTICAST_GROUP = "239.255.0.1"
+MULTICAST_PORT = 9999
+DISCOVERY_TIMEOUT = 3.0  # seconds to wait for server responses
+BEACON_INTERVAL = 2.0  # how often server announces itself
+
+# Game grid settings
+GRID_WIDTH = 40
+GRID_HEIGHT = 30
+CELL_SIZE = 20  # pixels
+
+# Game timing
+TICK_RATE = 0.1  # seconds (10 ticks per second)
+FPS = 60  # client rendering FPS
+
+# Game rules
+INITIAL_SNAKE_LENGTH = 3
+SNAKE_GROWTH = 1  # segments to grow when eating food
+
+# Colors (RGB)
+COLOR_BACKGROUND = (0, 0, 0)
+COLOR_GRID = (40, 40, 40)
+COLOR_FOOD = (255, 0, 0)
+COLOR_SNAKES = [
+    (0, 255, 0),    # Green - Player 1
+    (0, 0, 255),    # Blue - Player 2
+    (255, 255, 0),  # Yellow - Player 3
+    (255, 0, 255),  # Magenta - Player 4
+]
+
+# Directions
+UP = (0, -1)
+DOWN = (0, 1)
+LEFT = (-1, 0)
+RIGHT = (1, 0)
+
+OPPOSITE_DIRECTIONS = {
+    UP: DOWN,
+    DOWN: UP,
+    LEFT: RIGHT,
+    RIGHT: LEFT,
+}