[US-04] Playing alone against the computer

Description

Implements single-player mode with AI opponent for Battleship game.

Changes

• Added AI player interface with Easy and Normal difficulty levels
• Easy AI makes random valid moves
• Normal AI uses hunt mode strategy (targets neighboring cells after hits)
• Comprehensive test coverage for both AI implementations
• Follows layered architecture pattern from planning documents

Requirements Implemented

✅ Single-player mode with automated opponent
✅ Computer places ships in valid positions (no overlaps, within boundaries)
✅ Computer shoots at coordinates not previously targeted
✅ Easy difficulty: Random shooting behavior
✅ Normal difficulty: Hunt mode strategy (targets neighbors after hits)

Acceptance Criteria Validated

✅ When single-player mode starts, computer's fleet exists, is valid and hidden
✅ When computer makes a move, its shot is at coordinate not previously targeted
✅ In Normal mode, if shot hits, following shots try neighboring cells until ship is sunk

Technical Details

• Package: org.example.ai
• Classes: AIPlayer (interface), EasyAI, NormalAI
• Testing: Comprehensive unit tests with JUnit 5
• Architecture: Follows layered pattern with clear separation of concerns

Closes #4