Change-size indicator on versions

As a person reviewing a workflow's history, I want each version to show how much changed, so that I can gauge the size and risk of a release at a glance without opening it.

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

Outcome

Each version in the history shows a change-size summary, the workflow equivalent of a diff stat. This came out of the June design discussion, where the size of a change was called the most useful signal for someone governing a production workflow. It is deliberately deferred to Phase 2; Phase 1 ships versions with who, what, and when only.

Scope

• Compute a change summary between a version and its predecessor from their two snapshots.
• Define the unit for a DAG workflow: steps, triggers, and edges added, removed, or changed, and optionally a per-step code diff (added and removed lines).
• Surface it on each version row in the history.

Open decision

What to show: a simple count (steps and triggers changed), a detailed per-step code comparison, or both. Lean is to start with the count and add the detailed comparison later.

Key surfaces

The two snapshots either side of a version, the version history UI.

Estimate

5 story points.

Start with the two snapshots on either side of a version and the version history UI. Define the initial count-based summary for steps, triggers, and edges, then surface it on each version row. Done means the history shows a change-size indicator for every version, with the display choice resolved.