docs: rewrite README to cover wasip1/wasip2/wasip3 + browser polyfill suite

Restructure the README around the four subsystems the package now ships
(wasip1, wasip2, wasip3, browser). Update plugin tables with correct
export names, add the previously undocumented wasip2 plugins
(messaging, nn, sql, frame-buffer, graphics-context, surface, webgpu,
wasi-gfx), and document the browser/* capability set and subpaths.
Refresh the API reference, factories, and peer-dependency notes.

Author: Zachary Whitley