Native UI integration path: pugl + skia as an alternative to webview

Context

The UI story starts with webview. The open question is which native library to
integrate for people who want to render their own UI rather than ship HTML. The
goal is to let people bring their own UI library with an easy integration path.

This is a request from a community thread, tracked here so it is on record. It
is exploratory and not committed work.

Suggestion from the thread (ishineee)

Pair a windowing and event layer with a drawing library:

• pugl for the window and event loop. Supports macOS, Linux X11, and
  Windows. No iOS or Android today. pugl support for iOS looks addable if we go
  this route.
• skia for drawing. Backends for OpenGL, Metal, and Vulkan. The same drawing
  library used in Chromium, ChromeOS, and Android. Reachable from a native C API
  through skiasharp, which bundles it. The tradeoff is skia's build system, which
  is complicated to build yourself.

Prior art

• zigplug, a work-in-progress Zig plugin framework that recently integrated
  pugl: https://codeberg.org/bandithedoge/zigplug

Notes

• pugl covers desktop. Mobile would need extra work (the iOS pugl port above).
• skiasharp's bundled C API is the path to skia without wrestling its full build.
• Keep the integration seam generic so a different UI library can slot in.

Start by reviewing the proposed pugl window and event layer, skia drawing path through skiasharp's bundled C API, and the zigplug prior art. Define what a generic integration seam must support and how desktop and future mobile paths fit it; done means a concrete, agreed integration design rather than an exploratory option list.