Module plugin 

Plugin registration trait surface. Used by external plugin crates to register pipe factories without depending on the umbrella proxima crate. AppBuilder in the umbrella implements this trait so the existing plugin::register(app_builder) call sites keep working.

Traits

PluginRegistry

Surface a plugin can register against. Equivalent to the relevant subset of methods on AppBuilder in the umbrella proxima crate. Adding methods here is breaking; keep the surface minimal.