chore: inital commit

Split project into a Cargo workspace with two crates:

- core/ (noctua_core): UI-independent document library
- ui/ (noctua_ui): COSMIC application scaffold (empty)

justfile for workspace (run, run-cli, check, test, build)

core/src/document/raster.rs

@@ -0,0 +1,12 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+// src/document/raster.rs
+//
+// Raster image document model (JPEG, PNG, etc.).
+
+use std::path::PathBuf;
+
+#[derive(Clone, Debug)]
+pub struct RasterModel {
+    pub path: PathBuf,
+    pub dimensions: (u32, u32),
+}