refactor(core): reorganize module structure

core/src/document/model/mod.rs

@@ -0,0 +1,12 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+// src/document/model/mod.rs
+//
+// The three concrete document models Noctua understands.
+
+pub mod portable;
+pub mod raster;
+pub mod vector;
+
+pub use portable::PortableModel;
+pub use raster::RasterModel;
+pub use vector::VectorModel;