Unity 6.6 Alpha (6000.6.0a2): CS0619 compile errors in UnityTypeConverters.cs and GameObjectLookup.cs

[NVX60110]

Environment

• Unity: 6.6 Alpha (6000.6.0a2) DX12
• • Package branch: beta
• • • OS: Windows 11

Problem

Unity 6.6 Alpha broke both object lookup APIs simultaneously with CS0619 (obsolete-as-error), making the package fail to compile even on the beta branch. Note: #1051 / #1058 fixed Unity 6.5 compat but does not cover 6.6 Alpha.

Two files affected:

1. Runtime/Serialization/UnityTypeConverters.cs (line 418)

error CS0619: 'EditorUtility.InstanceIDToObject(int)' is obsolete: 
'InstanceIDToObject(int) is obsolete. Use EditorUtility.EntityIdToObject instead.'

2. Editor/Helpers/GameObjectLookup.cs (line 74)

error CS0619: 'EntityId.implicit operator EntityId(int)' is obsolete: 
'EntityId will not be representable by an int in the future. This casting operator will be removed in a future version.'

Root Cause

In Unity 6.6, Unity removed the implicit int → EntityId cast AND marked InstanceIDToObject(int) as a compile error in the same version. The existing #if UNITY_6000_3_OR_NEWER / #else guards don't have a path that compiles cleanly on 6.6+.

Both direct API calls (EntityIdToObject(instanceId) and InstanceIDToObject(instanceId)) are blocked at compile time. The correct migration is EntityId.FromULong(ulong) → EntityIdToObject(EntityId), but the affected code sites only have an int instanceId available, not a ulong.

Workaround

For each affected site, add a #if UNITY_6000_6_OR_NEWER guard using reflection to bypass the compile-time restriction (the method still exists at runtime):

#if UNITY_6000_6_OR_NEWER
    var method = typeof(EditorUtility).GetMethod(
        "InstanceIDToObject",
        System.Reflection.BindingFlags.Static | System.Reflection.BindingFlags.Public,
        null, new[] { typeof(int) }, null);
    return method != null ? (UnityEngine.Object)method.Invoke(null, new object[] { instanceId }) : null;
#elif UNITY_6000_3_OR_NEWER
    return EditorUtility.EntityIdToObject(instanceId);
#321     return EditorUtility.InstanceIDToObject(instanceId);
#723 ```

A proper long-term fix would be to store/pass `EntityId` (ulong) alongside `instanceId` at the call sites so `EntityId.FromULong()` can be used directly without reflection.

## Impact
Project enters Safe Mode on first open. Cannot enter Play Mode until resolved.