[18.0] [IMP] spreadsheet_oca: what-if scenarios with cell overrides

[dnplkndll]

Adds what-if scenarios: a named set of cell overrides applied to a copy of a
spreadsheet, so alternatives can be modelled without touching the original.

Depends on #99 — its two commits are shared and drop out once
that merges.

Non-mechanical points worth flagging

• The original is never mutated. action_apply_to_copy deep-copies
  spreadsheet_raw before writing any override, then creates a new record.
• Overrides are validated on save, not on run. A constraint checks the JSON
  parses, is an object, and that every key is a usable cell reference — so a
  malformed scenario fails where the user typed it.

AI-assisted (Claude Code); every change reviewed, tested, and owned by the author.