Completes the checkout-and-history CLI slice — merge is the sixth and final verb (after timeline, mv, undo/redo, extras, clean). Line-based three-way merge into the working tree with fossil-style conflict markers, a mtime-independent up-to-date/ancestry gate, and a commit that mints the primary-first two-token P card and refuses while any marker remains. 1629 unit + 307 e2e green; dst 6x1000. check-in: c2c837c5e1 user: dmestas tags: trunk