Incompatibilities

Two incompatibilities with lean spec:

- Lean spec uses SSZ to serialize
- Lean spec has a testing "mode" (requires test-config feature)

The first I think should be removed as the python code ideally does not know anything about ssz at all.

The second should be added and is similar to having a minimal config

Start by locating the Python serialization/configuration code and the existing minimal-config path. Compare its behavior with the Lean spec, checking where SSZ knowledge is introduced and how the test-config feature represents testing mode. Done means the Python code no longer depends on SSZ and supports the spec's testing mode with coverage for both configurations.

Written by the indexing model from the issue text.