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/layers.rs

@@ -60,7 +60,7 @@ impl OpenCADStudio {
                 } else {
                     lt.name.clone()
                 };
-                let art = crate::linetypes::extract_pattern(&lt.description);
+                let art = crate::io::linetypes::extract_pattern(&lt.description);
                 ui::properties::LinetypeItem { name, art }
             })
             .collect();