UI node API map

Contract

Go callers use the github.com/busdk/bus-gx/pkg/gx package. This patch exposes only the first render-tree API surface:

API Page Purpose
Node Shared interfaces Normalizes concrete values to validated virtual nodes.
Renderer, HTMLRenderer, and RenderHTML Shared interfaces Render validated nodes to deterministic escaped HTML.
VNode VNode Stores the normalized immutable node shape used by renderers and tests.
Text Text Creates escaped scalar text nodes.
Element Element Creates safe lowercase structural HTML nodes.
Fragment Fragment Groups ordered children without output wrappers.
Props Props Stores deterministic validated attributes.

Consequence

The API is library-only. It has no bus gx CLI, source parser, custom function components, component composition, event dispatcher, lifecycle runtime, or browser mount API in this patch.