[US-04] Single-player mode vs. computer opponent

Implementation Summary

This merge request implements single-player mode against a computer opponent for the Battleship game, as specified in issue #4.

Features Implemented

1. Computer Player Class (ComputerPlayer.java)

   • Automatic legal fleet placement (no overlaps/out-of-bounds)
   • Difficulty levels: Easy (random shots) and Standard (hunt-and-target strategy)
   • Smart targeting that avoids repeat shots
   • Hunting mode that targets adjacent cells after hits

2. Game Integration

   • Single-player game initialization in GameController
   • Computer turn execution in GameService
   • Difficulty selection support

3. Acceptance Criteria Validation

   • ✅ Computer fleet exists and is legal but hidden
   • ✅ Computer never targets same coordinate twice
   • ✅ Standard difficulty targets adjacent cells after hits

Technical Implementation

• Computer AI: Strategy pattern for different difficulty levels
• Fleet Placement: Random placement with fallback systematic placement
• Targeting: Efficient untargeted position tracking
• Hunting Mode: Adjacent cell targeting after successful hits

Testing Coverage

• Unit Tests: ComputerPlayerTest.java covers all acceptance criteria
• Integration Tests: GameServiceTest.java validates single-player mode integration
• Test Coverage: All acceptance criteria verified with specific test methods

Pipeline Status

✅ Pipeline #6858: SUCCESS

• Compile job: ✅ Success
• Test job: ✅ Success
• All tests passing: ✅ Verified

Closes #4