[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).

Key Features Implemented:

Core Persistence:

• GameState class with complete serializable game state
• GameService.saveGame() and GameService.loadGame() methods
• File version validation and error handling
• Support for both single-player and two-player modes

State Preservation:

• Complete match state including board size, fleet layouts, shots, sunk status
• Current player turn, difficulty/mode, elapsed timer value
• Hidden information remains concealed from opponent after load
• Game finished state and winner preservation

Validation & Error Handling:

• File version compatibility checking
• Corrupted/invalid file detection
• Graceful error reporting without modifying current game
• File format validation with checksum-like validation

Acceptance Criteria Coverage:

• ✅ AC1: Save/load round-trip preserves complete state (shot history, visible cells, sunk indicators, turn, difficulty, timer)
• ✅ AC2: Corrupted files show error and preserve current game
• ✅ AC3: Finished games remain finished after load with same outcome

Technical Implementation:

• Java serialization for game state persistence
• Comprehensive test coverage with integration tests
• Idempotent round-trip verification
• File validation with version checking

Files Modified/Created:

• src/main/java/org/example/model/GameState.java - Serializable game state
• src/main/java/org/example/service/GameService.java - Save/load operations
• src/test/java/org/example/integration/SaveLoadIntegrationTest.java - Acceptance criteria tests
• src/test/java/org/example/service/GameServiceTest.java - Unit tests
• src/test/java/org/example/model/GameStateTest.java - Model tests

Closes #5 (closed)