getFreshUserPreferences

Mint a brand new set of preferences: a fresh id and a presence colour, and nothing else.

Deliberately does not read (or write) any persistent storage, and deliberately leaves every other key unset so the editor's own defaults apply. A host that wants to own the preferences outright — to keep a per-origin setting from outranking its own theme, say — seeds its state from this and writes down the handful of settings it actually decides.