Document Chekov 0.1 behavior before the 0.2 refactor

[Freyskeyd]

Summary

Chekov 0.1 did not have a reproducible behavioral baseline for the 0.2 refactor. Fresh Cargo resolution also selected Actix derive releases that no longer compile with Actix 0.12.

This change:

• pins actix_derive to the compatible 0.6.0 release
• adds an in-memory test showing that a failed command leaves aggregate state and version unchanged and does not create a stream
• documents current command execution, persistence, replay, subscription, and concurrency behavior
• records the gaps and behavior changes that Chekov 0.2 needs to address deliberately