[US-02] Take turns firing with hit/miss feedback

Description

Implements turn-based firing system for Battleship game with hit/miss feedback.

Requirements Implemented

1. Enforce alternating turns - GameService manages turn switching
2. Let active player select target coordinate - Coordinate input system
3. Report 'hit' or 'miss' immediately - Shot result feedback
4. Prevent firing at same coordinate twice - Board tracking system

Acceptance Criteria Verified

• ✅ Given A1 has not been targeted, when player fires at A1, result shown and A1 marked as used
• ✅ Given A1 is already targeted, when player attempts A1 again, system blocks action
• ✅ Given a turn completes, when control passes, only next player can act

Files Created/Modified

• src/main/java/org/example/model/Board.java - Board logic with shot tracking
• src/main/java/org/example/model/Coordinate.java - Coordinate validation
• src/main/java/org/example/model/Game.java - Game state management
• src/main/java/org/example/model/Player.java - Player turn management
• src/main/java/org/example/service/GameService.java - Turn-based game logic
• src/test/java/org/example/model/BoardTest.java - Board shot validation tests
• src/test/java/org/example/model/GameTest.java - Turn management tests

Technical Implementation

• Turn-based state machine in GameService
• Coordinate validation and shot tracking in Board
• Player turn switching with validation
• Comprehensive test coverage for all acceptance criteria

Closes #2