pancsta/asyncmachine-go

AI Summary: Implement a new `OnChange()` function in the `asyncmachine-go` library's `Machine` struct. This function, when set, will be called whenever the active states of the machine change, providing a simple notification without requiring handlers or error arguments. This addresses the need for lightweight state change notifications beyond tracing.

AI Summary: The task is to modify the asyncmachine-go library to prevent the Heartbeat and Healthcheck functions from automatically triggering state transitions. This will likely involve altering the state machine's logic to exclude these specific functions from the transition process, potentially by adding conditional checks or flags.

AI Summary: Implement a D2 sequence diagram generator for the asyncmachine-go TUI debugger to visualize transition sequences, improving scalability for large state machines. The diagram should be generated as a `.d2` file and optionally as a PNG image, potentially integrating with the existing visualizer tool.