Hand-write sf_define_type in bootstrap.py FIXUPS (skipped from transpile
because the spec uses &rest params and empty-dict literals the transpiler
can't emit). Registers define-type via register_special_form. Adds
step_limit/step_count to PREAMBLE (referenced by sx_vm.ml/run_tests.ml).
172 assertions pass (test-adt). Full suite: 4280/1080 (was 4243/1117).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>