Add native2d feature: wgpu 2D viewport window inside Tauri process

Spawns a GTK-free winit window with its own wgpu surface from Tauri's
setup hook on a background thread, sidestepping the WebKitGTK surface
contention on Wayland. Reuses render2d's renderer via a shared demo
module. Opt-in behind the native2d cargo feature; default build
unaffected.

src-tauri/render2d/src/lib.rs

@@ -12,6 +12,7 @@
 // Standard-Build (`cargo test`/`cargo build` ohne Features) enthaelt nur die
 // GPU-freien Schichten und ist damit unabhaengig von einer Display-Session.
 
+pub mod demo;
 pub mod ortho;
 pub mod shaders;
 pub mod tessellate;
@@ -20,6 +21,7 @@ pub mod types;
 #[cfg(feature = "render")]
 pub mod gpu;
 
+pub use demo::{demo_scene, initial_view_box};
 pub use ortho::{compute_ortho_matrix, meet_scale, mm_to_device_px, Mat4};
 pub use tessellate::{compile_scene, triangulate, GpuGeometry, PX_PER_M};
 pub use types::{FillPolygon, Line, Outline, Point, Rgba, Scene, ViewBox};