Add register clearing between context switches

- for security reasons CPU core registers need to be cleared whenever execution shifts to a new box to avoid leakage of information
- a box can opt-out from register clearing for performance reasons in case privacy is not required

No files, tests, or entry points are identified. Begin by locating the context-switch implementation and the representation of boxes, then determine how register clearing and a per-box opt-out would be specified and tested. Done means registers are cleared on every switch unless the box explicitly opts out, with leakage-prevention behavior verified.

Written by the indexing model from the issue text.