fix(render_util): include <functional> for std::function

[dillona]

Compatibility:

patch

Issues/Tickets:

N/A

Release notes comment:

Add missing include to make Arena.h self-contained when using std::function.

Comments for the reviewer:

Single-header fix: include in Arena.h to avoid relying on transitive includes.

Look or scene setup change:

N/A

Special notes for production:

N/A

[jlanz]

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