fix: keep the shell running when a cancelled command is still loading (#506)

* fix: keep the shell running when a cancelled command is still loading

* fix: keep cancelled lazy commands single-flighted

* fix: release trusted scopes with the execution that owns them

* fix: clear a failed lazy load independently of waiter cancellation

* fix: release the trusted scope when a cancelled load is abandoned

* fix: make lazy-load waiters detachable when cancelled

* fix: own shared lazy-load settlement on the load, not the invocation

* fix: settle a shared load without invoking loader promise methods

* test: count body executions after a cancelled load

* docs: state the timeout cancellation fix in terms of impact