fix(uix): support native navigation controls

[cdd1037]

Summary

• expose native Stack, NavItem, and SplitView handles and C APIs
• teach the UIX UiPage factory/compiler to create and compose these controls
• support reactive text, selected, open, and active bindings
• document the new UIX controls and APIs

Motivation

The legacy UiLayout parser already understands these navigation controls, but the newer UIX/UiPage path could not create them. Applications therefore had to emulate navigation with generic containers and buttons.

Validation

• built the optimized Windows x64 DLL with clang-cl
• exercised the native controls in a UIX application using SplitView + NavItem + Stack
• verified native selected/hover rendering and page switching

Build: https://github.com/cdd1037/core-ui/actions/runs/32629855670