registry package

Package registry is spike (a) of tasarim-v6 §5.5: can a realm store an interface value handed over by ANOTHER realm and call it later, and what can that foreign code do to the registry while it runs?

It is deliberately tiny and is never part of the product. Two exports are intentionally unsafe probes and are marked as such: SharedBoard (returns a pointer into registry state) and UnsafeBump (a non-crossing exported mutator). Production core has neither; this spike measures why.