Webview Support (phase 1)

Enable use of the system webview to render high-fidelity web content.

First release will be limited to transient surfaces such as modal dialogs.

Start with issue #28 and its description of system webview support for transient modal surfaces. The payload names no files, tests, or entry points, so first clarify the phase-one scope and documentation changes required; done should be defined as a documented, reviewable description of the supported webview behavior.

Written by the indexing model from the issue text.