The transcript becomes a contribution area (transcript.directives). A plugin
registers a named directive and the model addresses it by emitting
::name{key="value"} as its own paragraph; that leaf renders as the plugin's
component, wrapped in the contribution error boundary. Unclaimed or malformed
directives stay plain prose, so nothing changes for text that merely looks
like a directive (std::vector) or for users with the plugin disabled.
Core ships ::preview{file="..."} as the reference consumer (the existing
preview-attachment card), the desktop platform hint teaches the model the
syntax, and the SDK exports the area + types so runtime plugin.js files get
the surface through the normal plugins API.
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