book: create-missing = true makes the 'SUMMARY.md references nonexistent file' condition unfalsifiable #5300

A book condition that cannot fail

contracts/apr-book-build-v1.yaml lists the falsification condition
"SUMMARY.md references nonexistent .md file". No check can trip it:
book/book.toml sets

[build]
create-missing = true

so mdbook build book writes an empty page for any SUMMARY.md entry with no file and exits 0. The book workflow runs mdbook build book before scripts/check_book_linkcheck.sh, so by the time the link check runs, the missing file exists on disk. A typo in a SUMMARY.md path ships as a blank chapter, and nothing goes red.

Falsifier

1. Add - [Ghost](ghost.md) to book/src/SUMMARY.md.
2. Run mdbook build book. It exits 0, and book/src/ghost.md now exists.

Fix options

• Set create-missing = false. mdbook then refuses a SUMMARY.md entry with no file, and the existing build step becomes the check.
• Or add a pre-build step that resolves every SUMMARY.md link against book/src/.

Either way, FALSIFY-BOOK-BUILD-001 (added under #5291) then covers the condition, and its prediction text can drop the caveat.

Refs #5291