fix(readme): restore the 458 lines my merge resolution dropped

The main merge at 25eb7225 resolved README.md's conflicts with a regex that matched only 4 of the
5 blocks and swallowed the text between them -- 1,887 lines became 1,429, taking the flag-count
sentence readmedriftcheck arm (B) reads, among much else. Redone as a real three-way merge
(git merge-file against the merge base) with a line-by-line resolver instead of a regex; all five
blocks resolved, gate counts renumbered to the union 557, 1,887 lines restored.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>