fix(unreal): macOS build failure — Nil macro collision with Objective-C++

[brougkr]

Summary

On macOS/Apple platforms, Objective-C++ defines Nil as a macro (#define Nil nullptr in objc/objc.h). Unreal Engine compiles all C++ as Objective-C++ on Apple platforms, so FSpacetimeDBUuid::Nil() in Builtins.h collides with this macro and produces:

error: expected member name or ';' after declaration specifiers; 'nullptr' is a keyword in Objective-C++

Every UE project using the SpacetimeDB Unreal SDK on macOS hits this build failure.

Affected platforms

• macOS (any version) — Unreal Engine 5.x compiles as Objective-C++ on Apple platforms
• Affects all SpacetimeDB Unreal SDK versions that expose FSpacetimeDBUuid::Nil()

Reproduction steps

1. Create or open any Unreal Engine project on macOS
2. Add the SpacetimeDB Unreal SDK (v2.1.0)
3. Build the project
4. Observe the build failure in Builtins.h at FSpacetimeDBUuid::Nil()

Fix

Added #pragma push_macro / #undef Nil / #pragma pop_macro guards in Builtins.h:

• Before struct definitions: Save and undefine the Nil macro so it doesn't interfere with FSpacetimeDBUuid::Nil() and any other identifiers using Nil
• After the last closing brace: Restore the original Nil macro so downstream Objective-C++ translation units are unaffected

This is the standard UE pattern for handling platform macro collisions (similar to how UE itself handles check, verify, TEXT, etc. macro conflicts).

Test plan

• Verify macOS UE build succeeds with the SDK included
• Verify Windows/Linux builds are unaffected (the #ifdef Nil guard is a no-op when the macro is not defined)
• Verify FSpacetimeDBUuid::Nil() and NilUuid() Blueprint functions work correctly at runtime

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