Minor.Frame Navigation Runtime

Add imperative frame-navigation runtime APIs and a link(href, { src, target, history }) mixin for declarative client navigations.

run() now initializes from run({ loadModule, resolveFrame }), the package exports navigate(href, { src, target, history }) and link(href, { src, target, history }), and components can target mounted frames via handle.frames.top and handle.frames.get(name). The link() mixin adds href/rmx-* attributes to anchors and gives buttons and other elements accessible link semantics with click and keyboard navigation behavior.