Publish prebuilt sample plugins for download

Trying the Plugin SDK's samples currently requires a C# toolchain: a user must install the .NET SDK, extract the SDK distribution, set OBJO_PLUGIN_SDK_ROOT, and build each sample from source before anything can be installed into Studio's Plugin Library. For someone who just wants to see a finished plugin working in Objo code, that is a lot of steps before the first line of code.

Publish the sample plugins as prebuilt, ready-to-install .objopackage downloads alongside the Plugin SDK release. Any Objo Studio user can then download a sample, install it into the Plugin Library, select it for a project, and write ordinary Objo code against it — with no C# knowledge and no build step. Pure-managed samples publish as one cross-platform file; samples carrying native libraries publish for the platforms they have been built and tested on, with the supported platforms listed at the download point.