Function define_plugin_sign 

pub fn define_plugin_sign(
    registry: &SignRegistryHandle,
    plugin_id: &str,
    name: &str,
    spec: SignSpec,
) -> String

The define-sign host-service body. Registers into the SAME registry native producers use, namespaced by plugin id so unload can reverse it.

Returns the registered (namespaced) name so the caller can record a teardown token.

Copy-on-write against the ArcSwap: definitions are written at load and read on the render path, so the write clones and stores rather than locking anything a frame might wait on.