Saxonberg Server API
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    PopulateSpec: string | { onto: string; template: string }

    One populates entry. A bare path string moves the spawned instance into self (containment); the object form places it on a surface — onto names another entry's path (a Surfaced host) that must appear earlier in the list (so it's already populated). The surface itself is populated by a bare-string entry first; its resting items follow with onto pointing at it.