kernel: Add removable bootstrap keep‑alive pin

[sirtimid]

Introduce a Kernel.pinVatRoot(vatName) method that external code (tests, control panel) can call to keep a vat or arbitrary kernel object alive to ensure that references (e.g., to bootstrap vats) that are only held externally (e.g., by the control panel or test scripts) but never explicitly exported via a message parameter are counted. An unpin helper can handle removal.

Ref:
#492 (comment)
https://github.com/Agoric/agoric-sdk/blob/master/packages/SwingSet/src/controller/controller.js#L459
https://github.com/Agoric/agoric-sdk/blob/master/packages/SwingSet/src/kernel/state/kernelKeeper.js#L1853