[US-04] Playing alone against the computer

Description

Implementation of single-player mode with computer AI opponent.

Changes

• Added ComputerPlayer class with AI logic for ship placement and shooting
• Implemented single-player game mode with computer opponent
• Added difficulty levels (Easy/Random and Normal/Strategic)
• Integrated computer AI with existing game state management
• Added comprehensive test coverage for computer AI behavior

Requirements Implemented

✅ Single-player mode where computer places ships correctly (no overlaps, within boundaries) ✅ Computer takes turns shooting at valid coordinates not previously used ✅ Different difficulty levels: Easy (random) and Normal (strategic targeting) ✅ Computer fleet is valid and hidden at game start

Acceptance Criteria Validation

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

Current Status

✅ Pipeline Status: SUCCESS - All 107 tests passing (0 failures, 0 errors, 1 skipped)

• Computer AI logic fully implemented and tested
• Game state transitions working correctly
• Shot validation and recording functioning properly
• Strategic targeting in Normal mode operational

Ready for merge - all validation criteria met.

Closes #4 (closed)