If it ever becomes necessary to scrap the entire core layer because of
a design flaw that's too difficult to work around, here's the
solution that provides a clean upgrade path:

-- insert a new s2layers row with a type of 'core' and a b2lid
   of 0, like the existing core layer.  but all core layers are
   required to have a 'majorversion' layerinfo field.

-- make the /customize/ UI let people pick in the layer drop down
   not just layers, but core+layer combos that work, labeled with
   their major versions, so people will prefer higher ones.

-- the ljviews code that gives the data structure to S2 runner will
   have to know which version the S2 code is expecting, and do the
   right thing, based on the core layer's 'majorversion' data.

-- the S2 Builtin layer has to be a superset every version higher.