Add layout.fitOnePage, a speculative typography search that removes one
page from the document while preserving relative font hierarchy.
Every size receives the same half-point-rounded reduction; line spacing
never falls below 0.94 of its original and body text never below 8.5 pt.
The command refuses on explicit page breaks, excessive final-page content,
formatting protection, tracked formatting and the cumulative 1 pt budget,
and leaves the document unchanged when it refuses.
Add the Layout ribbon button, KeyTip and a code-drawn icon, cover the
command with 41 tests, and regenerate the parity checklist.