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/mod.rs

@@ -1476,7 +1476,7 @@ pub enum Message {
     Noop,
     /// GitHub releases API returned a result. `Some(version)` means a
     /// newer release exists; we open the update-notice window.
-    UpdateCheckResult(Option<crate::update_check::UpdateInfo>),
+    UpdateCheckResult(Option<crate::io::update_check::UpdateInfo>),
     /// User dismissed the update-notice window.
     UpdateNoticeClose,
     /// First-launch default-association prompt: user accepted — register this
@@ -2072,7 +2072,7 @@ impl OpenCADStudio {
         s.main_window = Some(id);
         let open_main = open_task.map(|_| Message::Noop);
         let check_update = Task::perform(
-            crate::update_check::check_for_update(),
+            crate::io::update_check::check_for_update(),
             Message::UpdateCheckResult,
         );
         let focus_cmd = s.focus_cmd_input();