Render actual 3D geometry (cubes + ground plane) in the Viewport panel
using the existing mesh_shader.wgsl Blinn-Phong pipeline with orbit
camera controls (left-drag to orbit, middle-drag to pan, scroll to zoom).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Add full-frame-cycle integration test to dock.rs (16 tests total) and
update editor_demo to use DockTree layout instead of a single panel.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Interactive demo showing panel, text, button, slider, and checkbox
widgets rendered via the new UiRenderer pipeline. Uses winit event
loop with mouse input forwarded to UiContext.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
