The sandbox's context builder was a near-copy of the Node runtime's. Both
now come from createPluginContext in @yaakapp-internal/lib, with each
runtime supplying only a transport.
The two places hosts genuinely differ are optional transport methods:
`stream` (a window reporting navigation until it closes) and `form` (a
prompt that re-renders as values change). The sandbox has neither, so
openUrl refuses and a form is drawn once from its defaults.
Adds packages/plugin-sandbox: QuickJS-ng compiled to wasm, running in a
dedicated worker, with a runtime shell inside it that loads a plugin bundle
and answers the same InternalEventPayload events the Node runtime answers.
Plugins are unmodified.
Wires the browser host's template function, authentication, cURL import and
template render commands to it, and relaxes TemplateCallback's Send bound on
wasm32 so the engine's renderer can call back out to a plugin.
Gregory Schier