Add core state machine integration test

[panarch]

Summary

• add a new test covering state machine operations using the memory backend
• clean up the test by adding a dispatch! macro to send events and assert transitions

Testing

• cargo clippy --all-targets -- -D warnings
• cargo test -- --nocapture

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https://chatgpt.com/codex/tasks/task_e_684e5f0ad350832a8529cf265aef7f4f