fix(ui-navigation): correct cj/undo-kill-buffer off-by-one on plain invocation

(interactive "p") forces arg >= 1, so the old (if arg (nth arg ...) (car ...)) always took the nth branch and plain M-S-z re-opened the second-most-recently-killed file. Index with (nth (1- arg) ...) so a numeric prefix is 1-based and no prefix re-opens the most recent. The two undo-kill tests now exercise the real no-prefix path (arg=1 reopens the first) and a 1-based numeric prefix; both failed against the bug and pass after. Filed a follow-up for a separate delq-vs-delete skip-visited defect found in the same function.

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