feat(editor): add voltex_editor crate with IMGUI core (font, draw_list, widgets)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/voltex_editor/src/lib.rs

@@ -0,0 +1,10 @@
+pub mod font;
+pub mod draw_list;
+pub mod layout;
+pub mod ui_context;
+pub mod widgets;
+
+pub use font::FontAtlas;
+pub use draw_list::{DrawVertex, DrawCommand, DrawList};
+pub use layout::LayoutState;
+pub use ui_context::UiContext;