Clarifying old / reference / frozen terms

[max-sixty]

This is a bit refine-y, but:

In various parts of the code & docs, we use a bunch of different terms for "snapshot":

• Snapshot
• Value
• Contents

For the "existing snapshot value":

• Reference value
• Old snapshot
• Frozen value

For the "proposed snapshot value":

• Generated snapshot
• New snapshot

(I'm sure there are lots more, these just come to mind)

While not doing some big disruptive refactor, I'd propose we gradually coalesce on:

• "Existing" for the value that's in the repo
• "Generated" for the value that the code generated
  • Possibly "proposed" once it comes to reviewing the snapshot
• (and no view on "snapshot" / "value" / "contents")