refactor: move resource/IO root modules under io/

Relocate three root-level modules into io/, where they belong by concern:

- update_check.rs -> io/update_check.rs (its own doc-comment notes it
  mirrors how crate::io wraps blocking calls in async).
- patterns.rs, linetypes.rs -> io/ (standard resource catalogs parsed
  from asset files and populated into the document — IO-shaped work).

Fix the moved files' include_str! paths (one level deeper), add the
mod declarations to io/mod.rs, drop them from lib.rs/main.rs, and
re-point all crate:: references to crate::io::. No behavior change.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/update/file.rs

@@ -341,7 +341,7 @@ pub(super) fn on_open_file(&mut self) -> Task<Message> {
                 self.tabs[i].scene.selected = rustc_hash::FxHashSet::default();
                 self.tabs[i].scene.preview_wires = vec![];
                 self.tabs[i].scene.current_layout = "Model".to_string();
-                crate::linetypes::populate_document(&mut self.tabs[i].scene.document);
+                crate::io::linetypes::populate_document(&mut self.tabs[i].scene.document);
                 self.tabs[i].properties = PropertiesPanel::empty();
                 // Seed the current table / multileader style from the file's
                 // header so the ✓ marks the right one (text/dim/mline come from

src/app/update/mod.rs

@@ -537,7 +537,7 @@ impl OpenCADStudio {
                 let i = self.active_tab;
                 self.push_undo_snapshot(i, "CLEAR");
                 self.tabs[i].scene.clear();
-                crate::linetypes::populate_document(&mut self.tabs[i].scene.document);
+                crate::io::linetypes::populate_document(&mut self.tabs[i].scene.document);
                 self.tabs[i].properties = PropertiesPanel::empty();
                 let doc_layers = self.tabs[i].scene.document.layers.clone();
                 let vp_info = self.tabs[i].scene.viewport_list();
@@ -2449,7 +2449,7 @@ impl OpenCADStudio {
                 Task::none()
             }
             Message::UpdateNoticeOpenRelease => {
-                crate::sys::open_url(crate::update_check::RELEASES_PAGE);
+                crate::sys::open_url(crate::io::update_check::RELEASES_PAGE);
                 self.close_active_modal();
                 Task::none()
             }