[Feat/UI]: Implement Time-Travel Checkpoint Debugger with Snapshot Variable Diff Block

Problem

Operators tracing agent memory restoration chains lack an interface component to perform granular variable comparisons between an active runtime state tracking window and historical checkpoint snapshots pool [1.5].

Proposed Solution

Develop a high-performance code diff block layout component (StateDiffViewer) inside components/experimental/time-travel/:

1. The Checkpoint Timeline Selector: Render a clean clickable sequence node interface displaying individual checkpoint snapshots history hashes (e.g., hcp_v40, hcp_v41, rollback).
2. The Variable Modification List: Display precise structural code comparison text metrics indicating explicit delta properties (e.g., - model: gpt-4o -> + model: claude-4, - memory_tokens: 4200 -> + memory_tokens: 5100).

Use Case

Empowers distributed systems engineers to inspect state lineage mutations visually after executing a recovery routine, accelerating debug loop analysis workflows during enterprise cluster integration tests.

Start in components/experimental/time-travel/ and identify the UI entry point for StateDiffViewer. Review how checkpoint hashes and variable changes are represented before planning the timeline selector and variable modification list. Done means the component supports selecting snapshots and displays their variable deltas as described in the issue.