How do you stop AI coding agents from turning one bad change into a two-day debugging snowball?

A developer using Codex on a SwiftUI app encountered a significant debugging challenge when an AI agent's single bad change led to a performance regression. This issue wasn't immediately caught, and subsequent changes compounded the problem, resulting in animation skips, delayed taps, and lagging screen transitions. Reverting all changes was not feasible due to valid later modifications. The developer is now seeking effective guardrails, such as small checkpoints, isolated worktrees, automated performance tests, physical-device checks, or human review, to prevent similar 'debugging snowball' scenarios.