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/loader.rs

@@ -0,0 +1,72 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+// src/document/loader.rs
+//
+// Document loading: format dispatch and directory scanning.
+
+use crate::document::format::{detect_format, DocumentFormat};
+use crate::document::portable::PortableModel;
+use crate::document::raster::RasterModel;
+use crate::document::session::{SessionData, SessionItem};
+use crate::document::vector::VectorModel;
+use crate::document::{core::PageLayout, DocumentContent};
+use crate::error::{Error, Result};
+use std::path::Path;
+
+/// Loads a document from a path and returns its content.
+pub fn load_document(path: &Path) -> Result<DocumentContent> {
+    if !path.exists() {
+        return Err(Error::NotFound(format!("{:?}", path)));
+    }
+
+    match detect_format(path) {
+        Some(DocumentFormat::Portable) => {
+            // TODO: Replace with actual pdfium-render loading.
+            Ok(DocumentContent::Portable(PortableModel {
+                path: path.to_path_buf(),
+                page_count: 3,
+                layout: PageLayout::new(vec![(595.0, 842.0); 3], 10.0),
+            }))
+        }
+        Some(DocumentFormat::Vector) => {
+            // TODO: Replace with actual usvg parsing.
+            Ok(DocumentContent::Vector(VectorModel {
+                path: path.to_path_buf(),
+                view_box: None,
+            }))
+        }
+        Some(DocumentFormat::Raster) => {
+            // TODO: Replace with actual image-crate loading.
+            Ok(DocumentContent::Raster(RasterModel {
+                path: path.to_path_buf(),
+                dimensions: (1920, 1080),
+            }))
+        }
+        None => Err(Error::Unsupported(format!(
+            "extension: {}",
+            path.extension()
+                .and_then(|e| e.to_str())
+                .unwrap_or("(none)")
+        ))),
+    }
+}
+
+/// Scans a directory for supported files and populates the session.
+pub fn scan_directory_into(session: &mut SessionData, dir: &Path) {
+    if let Ok(entries) = std::fs::read_dir(dir) {
+        let mut paths: Vec<_> = entries
+            .filter_map(|e| e.ok())
+            .map(|e| e.path())
+            .filter(|p| p.is_file() && detect_format(p).is_some())
+            .collect();
+
+        paths.sort();
+
+        session.items = paths.into_iter().map(|p| SessionItem::new(p, 0)).collect();
+    }
+
+    session.current_index = if session.items.is_empty() {
+        None
+    } else {
+        Some(0)
+    };
+}