Move marshal initialization to beginning

Prepare to use options.sponsor to support scheduler infrastructure
actors.
Implement event buffer actor
Begin refactoring checkpoint protocol
Replace dispatchEvent with eventBuffer

account tests temporarily disabled
Checkpoint in endpoint/receptionist
Buffer output to transport
Remove sponsor from tart-marshal API
Apply effects directly in endpoint/receptionist
added restore-from-checkpoint test
Load actor state/behavior from external source (text)
Test structures with circular references
Ensure sequencing of init/start
Refactor test to use snapshots
Pluggable create/send + time/seq for events
Remove processed event and update actor state
Preliminary exception handling for snapshot
Reorganize implementation
Integrate snapshot with checkpoint
Restore snapshot during initialization
Isolate memento encoding