Document operations live in core, beside compile

Writing a document is a pure function over a DocumentVersion: core exports it beside compile, it takes a version and returns a new one, and the editor calls it exactly as a script would. setNodeProp is the first; the shipped shape is in the compile reference.

The studio has to write documents, and nothing said where the writes lived. With the studio as the only planned caller, the default home was inside it — which turns every write into an editor feature. Composition is the product and the editor is one driver of it, so a later caller — a CLI, a CI job, a fixture generator, an agent — would either reimplement the write, free to disagree about what a valid document is, or drive a browser to avoid doing so. The second caller is not hypothetical: the demo publishes hand-written fixture documents through a script, with no editor anywhere in the loop.

A separate @nubbin/authoring package was the close alternative, and its case had two halves. The weight half — that authoring code burdens consumers who only render — is measured dead: a render-only application takes nothing from core but types, which erase, so a production build of the demo contains none of core's runtime, and pure write functions vanish from such a build exactly as compile already does. The surface half — that core as contract-plus-authoring is a bigger thing to explain, version and support, and that semver binds the export list whether or not it costs bytes — was heard and lost: one package owning the definition of a valid write beats a shorter export list. Keeping the operations studio-private was rejected outright: it is free until a second writer appears, and the second writer had already shipped.