Model the multi-step form as one tagged union: step/errors exist only while
Editing, and Submitting/Submitted/Failed carry a parsed Valid payload. So
"submitting while a field is invalid" and "success screen with errors set"
are unrepresentable by construction.
Pure transitions (next/back/submit/resolve) with a spec covering the key
invariants; illegal events are no-ops. The page becomes pure composition.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
