Export RenderFragments through native dynlib

[elcritch]

Summary

• export RenderFragments, handles, cursors, and operations directly through the generated native ABI
• add imageStyle(ImageId, Fill) and preserve custom fills for ImageRef
• share Siwin frame setup and fallback policy between Renders and RenderFragments
• require Binny 0.5.22 for opaque generic sequence/table layouts

Testing

• nim build_dynlib
• nim r -d:useNativeDynlib tests/tdynlib_api.nim
• nim r -d:useNativeDynlib tests/tdynlib_renderfragments.nim
• dynlib compile: examples/siwin_renderfragments.nim and examples/siwin_replace_image.nim
• normal compile: examples/siwin_renderfragments.nim

Reviewed with an Astra xhigh subagent; its Siwin-policy finding was addressed by routing both input types through the same implementation.