[US-02] Alternating shooting with feedback

Description

Implements alternating shooting functionality for Battleship game with turn management, hit/miss feedback, and duplicate shot prevention.

Changes

• Core Models: Added ShotResult enum, ShotTracker, GameState for turn management
• Game Logic: Implemented shooting functionality in GameService with turn switching
• UI Integration: Updated Main.java with shooting phase and turn-based gameplay
• Testing: Comprehensive test suite covering all acceptance criteria

Acceptance Criteria Verified

✅ When firing at A1 for the first time, a message should appear showing hit or miss, and that field should become marked.

• Tested in GameServiceShootingTest.testPlayerShootingHit() and testPlayerShootingMiss()

✅ When the same field is targeted again, the shot should not go through, and the next player should then act.

• Tested in GameServiceShootingTest.testPlayerShootingAlreadyShot()

✅ After one shot cycle finishes, the control changes so that only the other player can continue.

• Tested in GameServiceShootingTest.testAlternatingTurns()

Technical Implementation

• Turn-based shooting system with proper state management
• Shot validation and duplicate prevention
• Hit/miss detection with ship damage tracking
• Game end detection when all ships are sunk
• Comprehensive test coverage with 25+ test methods

Dependencies

• Builds upon ship placement functionality from Issue #1 (closed)
• Follows layered architecture pattern from planning documents

Closes #2