Read a tree's items in compiled code from a run the frame keeps, start a run at the finger's leaf, and move indices only for a list an index refers to (021)

- The frame keeps the run compiled code read a tree's item from last (the
  list as it was: its version); an item it holds is read from its column,
  else pure_run_item takes the item's run and steps.
- pure_list_run starts at the leaf the finger keeps when it holds the
  position (a slice found by key just before): no second descent; the
  run after it is found from the root (PureRun.pathless).
- LIST_APPEND and LIST_REMOVE move index states only for a list one
  refers to.
- A search (core only): 89 ms compiled (answers the executor's).

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