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

Implementation Summary

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

Requirements Implemented

✅ 1) Offer Single-Player: the computer places a legal fleet (no overlaps/out-of-bounds, standard sizes)

• ComputerPlayer.placeFleetAutomatically() method handles automatic ship placement
• Validates all placements to ensure no overlaps and within bounds
• Uses standard fleet sizes: Carrier (5), Battleship (4), Destroyer (3), Submarine (3), Patrol Boat (2)

✅ 2) On its turn, the computer shall select a valid untargeted coordinate and fire; at minimum it shall avoid repeats

• ComputerPlayer.generateShotPosition() ensures all shots are at untargeted positions
• Both Easy and Standard difficulty levels avoid duplicate shots
• Comprehensive test coverage for shot validation

✅ 3) Provide difficulty options: Easy (random shots) and Standard (after a hit, preferentially target adjacent cells until the ship is sunk)

• Difficulty enum with EASY and STANDARD levels
• Easy: Pure random shot selection
• Standard: Hunting strategy that targets adjacent cells after hits
• Smart targeting logic with potential target tracking

Acceptance Criteria Validated

✅ AC1: Given Single-Player is chosen, when the match starts, then the computer's fleet exists and is legal but hidden

• Tested in testPlaceFleetAutomatically()
• Computer fleet placement validated for legality
• Fleet remains hidden from human player

✅ AC2: Given the computer took a turn, when the history is inspected, then its shot is at a previously untargeted coordinate

• Tested in testNoDuplicateShots() and testEasyDifficultyRandomShots()
• Both difficulty levels verified to avoid duplicate shots
• Shot history tracking validated

✅ AC3: Given Standard difficulty and a computer hit, when subsequent computer turns occur, then at least one subsequent shot targets an adjacent cell until the ship is sunk

• Tested in testStandardDifficultyAdjacentTargeting()
• Hunting mode activation after hits verified
• Adjacent cell targeting strategy validated

Technical Implementation

Core Classes:

• ComputerPlayer: Extends Player with AI logic for ship placement and targeting
• Difficulty: Enum defining EASY and STANDARD difficulty levels
• Game: Enhanced with single-player mode constructor and computer turn execution

Key Features:

• Automatic legal fleet placement for computer
• Difficulty-based shot generation
• Hunting strategy for Standard difficulty
• Turn-based gameplay with computer opponent
• Comprehensive test coverage for all acceptance criteria

AI Strategy:

• Easy: Pure random selection from untargeted positions
• Standard: Random initially, switches to hunting mode after hits
• Hunting mode targets adjacent cells systematically
• Potential target tracking for efficient ship sinking

Test Coverage:

• Unit tests for ComputerPlayer AI logic
• Integration tests for single-player game flow
• Acceptance criteria validation tests
• Edge case handling (corner positions, duplicate shots)

Closes #4