7. Extensions

Anything not covered by the core schema goes in an extensions object, present at every level (document, tree, node, edge). Keys MUST be namespaced by reverse-DNS or product slug, e.g.:

"extensions": {
  "aspen-grove": { "localId": "a7x2" },
  "loomsidian": { "unread": false, "collapsed": true }
}

Rules:

Importers MUST preserve extensions they don't understand across re-export.
Core semantics MUST NOT depend on extension content — a reader ignoring all extensions sees a correct tree.
Per-viewer UI state (collapsed, unread, hoisted, scroll position) MUST NOT appear outside extensions.