Fix Unity 6.4 Object ID compatibility

[RuiCarvalhoCouto]

Summary

• Fixes CS0619 compilation errors on Unity 6.4 and later.
• Uses Object.GetEntityId() on Unity 6.4+.
• Retains Object.GetInstanceID() on older supported Unity versions.
• Preserves the existing GsplatResourceManager.Release(int) public API.
• Stores internal object identifiers as ulong so both APIs share a common key type.

Testing

• Imported a model of 10M Gaussians & SH2.
• Rendered a model of 10M Gaussians & SH2 on Unity 6.3 LFS, in Play Mode and in Scene Mode.
• Rendered a model of 10M Gaussians & SH2 on Unity 6.5, in Play Mode and in Scene Mode.
• Check if SH3 and SH4 are rendering correctly.

[wuyize25]

Thank you for the PR and testing!