Enforce a top-down order of execution

Is it possible to force a top-down order for cell execution? New cells to execute would only be at the bottom of them notebook and existing cells would essentially be immutable. Maybe if a user wanted to edit or re-execute a cell, a copy could be made at the bottom and that new cell could be executed.

Doing this makes a notebook reproducible. The console has this behavior and its definitely nice being able to know when and what code was run.

The issue names no files or tests; start by reviewing how notebook cells are executed and how console execution differs. Clarify the proposed top-down, immutable-cell behavior and its reproducibility requirements before identifying the affected entry points and tests. Done requires an agreed design and implementation scope.

Written by the indexing model from the issue text.