## Summary
Change code-mode stored value updates to merge writes by key instead of
replacing the session's complete stored-value map after each cell
completes.
Previously, each cell received a snapshot of stored values and returned
the complete resulting map. When multiple cells ran concurrently, a
later completion could overwrite values written by another cell because
it committed an older snapshot.
This change moves stored-value ownership into `CodeModeService`:
- Each runtime starts from the service's current stored values.
- Runtime completion reports only keys written by that cell.
- The service merges those writes into the current stored-value map on
successful completion.
- Core no longer replaces its stored-value state from a cell result.
As a result, concurrently executing cells can update different stored
keys without clobbering one another.
The move into CodeModeService is motivated by a desire to have this
lifetime tied to a new lifetime object on that side in a subsequent PR.
f94157a4b2