What changed

• tightened validate_aggregation so custom aggregation callables must be invokable with a single positional argument (the iterable of node inputs)
• replaced the previous positional-parameter presence check with inspect.Signature.bind validation using one positional argument
• added a regression test (test_bad_add4) to reject two-required-argument aggregation functions at registration time

Why

• aggregation functions are always called with one argument in runtime paths (aggregation(node_inputs))
• before this change, functions that required two positional arguments could be registered and only fail later during network execution
• failing fast at config/registration time provides clearer feedback and prevents hard-to-debug runtime errors

Testing

• ✅ source .venv/bin/activate && pytest -q tests/test_aggregation.py
• ✅ source .venv/bin/activate && pytest -q