No way to put back an earlier save point in a draft

As a developer building a workflow, I want to put one of my own earlier save points back, so that I can undo a bad hour's work the same way I can undo a bad release.

Part of #4852 (Sandbox DevX epic). Phase 2, alongside restore from a version (#4864).

Outcome

The version list in a draft, and in a sandbox, offers to put an earlier save point back, the way the live workflow's list offers to put an earlier release back.

Today restore exists only on the live list, and the server finds the version by its release number. A draft has no releases, so there is nothing for it to find. The developer can look at every save point they have made and step back through them, and then has no way to say "that one, please". Which is the moment they most want it: they have broken something in the last twenty minutes and the good copy is sitting right there in the list.

Scope

• Restore a snapshot by its own number rather than by a release number, since that is the numbering a draft has.
• Offered from the save-point list, the same place the live workflow offers it.
• Same shape as the live one: it writes the old content in as new content and moves forward, rather than rewinding. Nothing in between disappears.
• No release recorded, because a draft publishes nothing.

Notes

Found while checking the draft experience against its design. Everything else on that list held; this was the gap.

Worth doing together with a look at the wording. "Restore" reads right for a release. For a save point it may want to be plainer.

Start with the draft and sandbox save-point list, then compare it with the live workflow version list where restore already exists. Trace the server lookup that currently uses a release number and identify how snapshots are numbered for drafts. Done means an earlier draft or sandbox save point can be selected, written as new content without deleting intervening saves, and without recording a release.