[US-05] Save, load, and resume a match

Implementation Summary

This merge request implements the save/load functionality for the Battleship game as specified in issue #5 (closed).

Features Implemented:

1. SaveLoadService - Core service for saving and loading game states

   • Serializes complete game state to files
   • Validates file paths and permissions
   • Handles error cases gracefully
   • Provides clear error messages

2. GameState - Complete serializable game state

   • Captures all game information: board size, fleet layouts, shots, sunk statuses, current player, difficulty/mode, timer
   • Preserves hidden information (opponent's concealed layouts)
   • Version validation for compatibility
   • Complete state validation

3. PlayerState & ShipState - Nested state classes for serialization

   • Player state: name, ships, shots fired, active status
   • Ship state: type, position, orientation, hit positions
   • Bidirectional reconstruction (state → object)

4. Comprehensive Testing

   • Unit tests for SaveLoadService (12 tests)
   • Unit tests for GameState (11 tests)
   • Integration tests for save/load functionality
   • Error handling tests for corrupted/invalid files

Requirements Validation:

✅ Requirement 1: Persist complete match state including:

• Board size (10x10)
• Both fleets' concealed layouts
• All shots and results
• Sunk statuses
• Current player turn
• Difficulty/mode
• Elapsed timer value
• Hidden information remains hidden

✅ Requirement 2: Load and restore exact externally observable state

• Boards, indicators, whose turn, timer, difficulty/mode
• Shot history and results preserved
• Visible cell states maintained

✅ Requirement 3: File validation and error handling

• Missing file detection
• Invalid/corrupted file detection
• Incompatible version detection
• Current game remains unchanged on error

✅ Requirement 4: Idempotent round-trip

• Save immediately followed by load preserves all observable state
• No changes to match state after round-trip

✅ Requirement 5: User file selection

• File path validation
• Directory existence checks
• File permission validation

Acceptance Criteria Validation:

✅ AC1: Mid-game state preservation

• Test fixture with complex game state
• Round-trip verification of all state elements
• Shot history, visible cell states, sunk indicators, turn, difficulty, timer

✅ AC2: Error handling for corrupted files

• Tests for corrupted file detection
• Tests for wrong file format detection
• Current game state preservation on error

✅ AC3: Finished game state preservation

• Win/loss outcome preservation
• Input disabling after load
• Game remains finished with same outcome

Technical Details:

• Serialization: Java Object Serialization for reliable state capture
• Validation: Comprehensive state validation before/after operations
• Error Handling: Clear, user-friendly error messages
• Testing: 84 total tests - ALL PASSING ✅
• Coverage: JaCoCo coverage reporting enabled

Current Status:

• Pipeline: ✅ SUCCESS (Pipeline #6884)
• Tests: ✅ 84/84 tests passing
• Compilation: ✅ Successful
• Ready for Merge: ✅ All validation criteria met

Closes #5 (closed)