Show what changed between two versions

As a person responsible for a live workflow, I want to see what changed between two versions, so that I can tell what a release actually did without opening both and reading them side by side.

Part of #4852 (Sandbox DevX epic). Phase 2, builds on version recording (#4866).

Outcome

Picking a version in the history shows what is in it: which steps were added, removed or changed, which triggers moved, which credentials were swapped. The content is already there, since every version points at the workflow exactly as it was published.

The commonest question about a live workflow is what changed on the day it started failing. Today the history answers who published it and when, and stops. The only way to the rest is to open both versions and compare them by eye, which nobody does at three in the morning.

Scope

• A comparison between a version and the one before it, and between any version and what is live now.
• Shown where the version is, in the editor's version list.
• Read-only. Restoring is a separate action and already exists (#4864).

Notes

This is the sibling of #4931, which shows how much changed. That one is a size, this one is the substance, and the two want the same comparison underneath.

There is already a component that renders a workflow difference, built for the AI assistant's suggestions. Worth looking at before writing a second one.

Start with the editor's version list and locate the existing component that renders a workflow difference for the AI assistant. Trace how versions point to published workflows and how the comparison can support a version against its predecessor or the live version. Done means a read-only comparison shows added, removed, and changed steps, moved triggers, and swapped credentials.