I really like the approach to time-travel testing here. Injecting the clock directly into the BTContext is a very clean way to avoid flaky CI tests without having to mock global time interfaces.
Quick technical question: When a node returns 0 (Running), does the Supervisor strictly adhere to the fixed tick interval (like the 100ms in the example), or is there any built-in exponential backoff? I'm wondering how it handles CPU load if you have hundreds of trees just waiting on I/O. Really clean architecture, great work!