Fix Unity 6.5 SceneHandle and Object ID compile errors

[Scrolen]

Summary

This PR fixes Unity 6.5 compilation errors caused by obsolete Unity APIs that are now treated as compile-breaking errors.

Unity 6.5 reports direct Object.GetInstanceID() usage as obsolete and recommends GetEntityId(), and deprecates implicit SceneHandle conversions to/from int in favor of GetRawData() and FromRawData().

Changes

• Added UnityCompatibility helper methods for Unity-version-safe access to:
  • UnityEngine.Object runtime IDs
  • raw SceneHandle values
  • SceneHandle construction from raw data
• Replaced obsolete SceneHandle implicit int conversions with raw handle access.
• Updated scene lookup/comparison code to use raw scene handles.
• Updated unloaded scene tracking to avoid obsolete handle conversions.
• Removed reliance on GetInstanceID() < 0 for observer condition cleanup and replaced it with explicit runtime-instance tracking.

Validation

Tested in Unity 6000.5.0f1.

Validation performed:

• Project compiles with zero errors.
• Host/client connection succeeds using Tugboat.
• Client disconnect/reconnect succeeds.
• Global network scene loading works.
• Global network scene unloading works while clients are connected.
• Late-joining client correctly receives currently loaded global network scene.