fix(devx): answer an unbuilt closure with exit 3 PREREQUISITE NOT MET, once

check:published-readme-exports discovered the unbuilt-closure precondition once
per README import, so an unbuilt worktree answered exit 1 -- the code reserved
for "a README is wrong" -- under 198 `Build first` lines naming READMEs the
reader never touched. The precondition is now detected up front, before the type
surface is built and before one document is judged, and answered once with the
sibling banner and exit 3.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012zGPuVVX3deAx9LdjK8jCk

Author: claude