Fix RuntimeInitializeOnLoadMethod for generic singletons in Unity 6.3

[Copilot]

Description

Unity 6.3 rejects [RuntimeInitializeOnLoadMethod] in generic classes. Both RuntimeSingleton<T> and ScriptableObjectSingleton<T> used this attribute on their ClearInstance() methods, causing compilation failures.

Solution

Moved the attribute to non-generic registry classes that invoke registered clear actions:

• RuntimeSingletonRegistry: Holds [RuntimeInitializeOnLoadMethod] and clears all RuntimeSingleton<T> instances
• ScriptableObjectSingletonRegistry: Holds [RuntimeInitializeOnLoadMethod] and clears all ScriptableObjectSingleton<T> instances

Each generic singleton registers its ClearInstance method during static initialization:

public abstract class RuntimeSingleton<T> : MonoBehaviour where T : RuntimeSingleton<T>
{
    static RuntimeSingleton()
    {
        RuntimeSingletonRegistry.Register(ClearInstance);
    }
    
    internal static void ClearInstance() { /* ... */ }
}

The registry uses a HashSet<Action> for automatic deduplication and invokes all registered actions at runtime initialization:

internal static class RuntimeSingletonRegistry
{
    private static readonly HashSet<Action> _clearActions = new();

    [RuntimeInitializeOnLoadMethod(RuntimeInitializeLoadType.BeforeSceneLoad)]
    private static void ClearAllInstances()
    {
        lock (_clearActions)
        {
            foreach (Action clearAction in _clearActions)
                clearAction.Invoke();
        }
    }
}

Thread-safe registration by locking directly on the collection. No reflection used.

Related Issue

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactor (code change that neither fixes a bug nor adds a feature)

Checklist

• I have added tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have updated the CHANGELOG
• My changes do not introduce breaking changes, or breaking changes are documented

Original prompt

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This section details on the original issue you should resolve

<issue_title>[Bug]: InitializeOnLoad is Broken in Unity 6.3 (for Generic Classes)</issue_title>
<issue_description>### Description

ClearInstance in generic class — This is in the com.wallstop-studios.unity-helpers package (v3.1.7) in RuntimeSingleton and ScriptableObjectSingleton. Unity 6.3 now rejects [RuntimeInitializeOnLoadMethod] in generic classes. This requires a package update.

Steps to Reproduce

1. Use Unity Helpers in a Unity 6.3 project

Expected Behavior

It works

Actual Behavior

It doesn't work

Unity Version

6000.x

Package Version

3.1.6

Platform

• Editor
• Windows
• macOS
• Linux
• iOS
• Android
• WebGL
• Other

Additional Context

No response</issue_description>

<agent_instructions>Figure out if we can keep the instance clearing, maybe by one more level of indirection. Ideally it is not done using reflection.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes [Bug]: InitializeOnLoad is Broken in Unity 6.3 (for Generic Classes) #190

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[wallstop]

@copilot Looks pretty good, left some minor feedback.

[wallstop]

@copilot Need to make the header dates correct