RFC: `:s/old/new/[gc]` find-and-replace

[garritfra]

Goals

• :s/old/new/ — replace first match in the current cell.
• :s/old/new/g — replace all matches in the current cell.
• :%s/old/new/g — replace all matches in every cell of the sheet.
• '<,'>s/old/new/g — replace within the last visual range (depends on gv-style range tracking).
• c flag — confirm each replacement interactively.

Open questions

• Regex or literal? Vim defaults to magic regex. Suggest starting with literal substrings + later adding \V / regex variant. Less footgun, easier to test.
• Formulas. Replace inside raw (so =A1+1 → =A1+2 for :s/1/2/g)? Probably yes, with the dep graph re-registering after each replacement. But — match what users expect for :%s/A1/A2/g carefully; this is not the same as a "rename column" operation and we shouldn't pretend it is.
• Undo granularity. One undo entry per :s invocation, not per cell. Needs a begin_batch / end_batch wrapper around the existing undo stack.
• Iteration order. Row-major. Document it.
• Confirm prompt UX. A modal-ish prompt at the bottom: replace with new (y/n/a/q/l), mirroring vim.

Out of scope

• Visual-block-scoped replace.
• :s inside a recorded macro (depends on the macros RFC).