Add Cysharp/R3 reactive programming library with ReactiveProperty demo

[Copilot]

This PR integrates the Cysharp/R3 reactive programming library into the Unity project template and demonstrates its usage with a practical example in the main scene.

What's Added

Package Integration

• Added com.cysharp.r3 dependency to Unity Package Manager via Git URL
• Updated the runtime assembly definition to reference R3.Unity
• Used the recommended UPM Git URL approach: https://github.com/Cysharp/R3.git?path=src/R3.Unity/Assets/R3.Unity

Demo Implementation

The demo is integrated into the existing Scene01_Intro.cs MonoBehaviour and showcases reactive programming patterns:

// R3 ReactiveProperty field
private ReactiveProperty<int> _reactiveScore = new ReactiveProperty<int>(0);

// Subscription in Start() method
_reactiveScore.Subscribe(score => 
{
    Debug.Log($"[R3 Demo] Reactive Score changed to: {score}");
});

// Automatic updates when score changes
public int Score 
{
    set
    {
        _score = value;
        HudUI.ScoreLabel.text = $"Score: {_score:000}/{ScoreMax:000}";
        
        // R3 ReactiveProperty Demo: Update reactive score
        _reactiveScore.Value = _score;
    }
}

How It Works

When players jump in the game (spacebar), the score increases and the reactive property automatically notifies all subscribers. The console will display:

[R3 Demo] Reactive Score changed to: 1
[R3 Demo] Reactive Score changed to: 2
...

This demonstrates the core reactive programming pattern where data changes automatically notify observers without manual event handling.

Best Practices Implemented

• Proper Memory Management: Added disposal in OnDestroy() to prevent memory leaks
• Unity 6.x Compatibility: All code is compatible with Unity 6000.2.0f1
• Minimal Integration: The demo seamlessly integrates with existing game mechanics
• Clean Code: Follows the project's existing coding standards and structure

The implementation showcases how R3 can simplify event-driven programming in Unity applications by eliminating the need for manual event subscription/unsubscription patterns.

[!WARNING]

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This pull request was created as a result of the following prompt from Copilot chat.

Add Cysharp/R3 as a package dependency to the Unity project.

• Create a new branch for this work.
• Add the R3 package using the recommended Unity Package Manager method (UPM via Git URL).
• Ensure the project compiles successfully after the addition.
• Demonstrate usage: Add a Cysharp.R3.ReactiveProperty instance to any existing MonoBehaviour subclass (for example, Scene01_Intro.cs). Implement a simple value assignment and subscription to show that it works (e.g., log a message when the property changes).
• Include all necessary using statements and ensure the demo code is Unity 6.x compliant, matching the ProjectVersion.txt.
• Provide a summary of changes in the PR description.

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