Literal case-sensitive substring search sharing the : command line's
editing (cmd_prompt discriminates the prompt; Enter dispatches). Jumps
on Enter only — incremental caret-chasing loses to the e-ink refresh
cost, same call as the snippet no-popup decision. Wraps with a notice,
keeps the caret on a miss, bare / repeats the editor-global last
pattern. n/N live in move_by so counts and Visual extension come free;
they are not operator targets.
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