initial working demo sans networking

Signed-off-by: Sienna Meridian Satterwhite <sienna@r3t.io>

crates/app/src/lib.rs

@@ -0,0 +1,12 @@
+//! Replicated cube demo app
+//!
+//! This app demonstrates real-time CRDT synchronization between iPad and Mac
+//! with Apple Pencil input controlling a 3D cube.
+
+pub mod camera;
+pub mod cube;
+pub mod debug_ui;
+pub mod rendering;
+pub mod setup;
+
+pub use cube::CubeMarker;