Add config default helper

[nclack]

Summary

• add damacy_config_validate_with_defaults() to expand numeric default knobs before create
• tighten validate_config/damacy_create so zero numeric defaults are rejected without the helper
• make Python and bench paths pass explicit defaults and update focused tests

Tests

• ninja -C build
• env UV_CACHE_DIR=/tmp/uv-cache ctest --test-dir build --output-on-failure
• env UV_CACHE_DIR=/tmp/uv-cache uv run --extra dev pytest -v python/tests/test_damacy.py::test_config_validates_eagerly python/tests/test_damacy.py::test_native_pipeline_rejects_out_of_range_enums