feat(editor): add editor_demo example with IMGUI widgets

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>

examples/editor_demo/Cargo.toml

@@ -0,0 +1,15 @@
+[package]
+name = "editor_demo"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+voltex_platform.workspace = true
+voltex_renderer.workspace = true
+voltex_editor.workspace = true
+wgpu.workspace = true
+winit.workspace = true
+bytemuck.workspace = true
+pollster.workspace = true
+env_logger.workspace = true
+log.workspace = true