Context & Objective
The implementation for the async step-down feature is already up in PR #14169. Because this mechanism introduces significant concurrency complexity and multiple synchronization points, we need to explicitly document what we have built.

The goal of this document is to establish a shared understanding of the synchronization model, systematically validate the concurrency assumptions made in the PR, and prevent subtle bugs (e.g., race conditions, ordering issues) during future maintenance.

Scope
Focus strictly on layered cursor operations. (Schema operations are out of scope).