Fourth and final family extraction. A pure behaviour-preserving move: the mutating checkout verbs -- update, bisect, merge, revert, undo, redo, commit, amend -- and the helpers private to them (the undo journal, the dirty-tree gate, the checkout record, the commit and amend manifest assembly) move verbatim into src/cmd/checkout.zig, byte-identical to their source but for the pub the dispatch forces on the eight handlers. commitEpoch stays in main.zig because main itself builds the ambient clock from it; shared helpers stay pub and aliased. With this, main.zig holds only main, the verb dispatch, init, the shared helpers and the test surface -- the eleven-thousand-line file is now four cmd families plus a thin dispatch core. No helper duplicated, no cross-cmd import, test count unchanged. check-in: ae54e47b3c user: dmestas tags: trunk