The C3 runtime SDK download for plugins and behaviors is now available. The addon downloads now include C3 runtime ports to demonstrate how they work. Find the downloads here: https://www.construct.net/make-games/manuals/addon-sdk
The initial version of the C3 runtime API documentation is available here: https://www.construct.net/make-games/manuals/addon-sdk/runtime-reference
The runtime has a very large API surface. The documentation does not yet cover everything, but it should be a good start.
For more information on how to make an addon for the C3 runtime, please see the new section on Runtime scripts in the guide. This is not yet 100% complete - it still needs to cover how to access DOM features when hosted in a worker, but there's no way to test that yet, so it's not covered for the time being. Please also note the section about use of private or undocumented API calls; you should not use these, we won't provide support for them, and we can't guarantee they will be kept backwards-compatible.
Hopefully this is enough to allow addon developers to start porting to the new C3 runtime. If you have any questions about the new C3 runtime SDK, or need any particular areas that aren't covered at all to be documented, please let me know!