Enable Phase 21 Multi-Fabric verification test

[t81dev]

This PR enables the Phase 21 Multi-Fabric verification test in the Makefile, which was previously missing from the build targets. It verifies that the existing test tests/test_phase21_multi_fabric.c compiles and runs successfully. The test validates multi-fabric orchestration and economic metrics reporting.

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PR created automatically by Jules for task 16983154404623583794 started by @t81dev

[google-labs-jules]

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