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/io/mod.rs

@@ -11,6 +11,9 @@ pub mod print_to_printer;
 pub mod step;
 pub mod stl;
 pub mod xref;
+pub mod linetypes;
+pub mod patterns;
+pub mod update_check;
 
 use crate::scene::DerivedCaches;
 use acadrust::entities::{Dimension, EntityType};
@@ -879,7 +882,7 @@ mod layer_roundtrip_tests {
     // round-trip through `ext`, and return whether every one survived.
     fn roundtrip_layers(ext: &str, count: usize) -> bool {
         let mut doc = CadDocument::new();
-        crate::linetypes::populate_document(&mut doc);
+        crate::io::linetypes::populate_document(&mut doc);
         let names: Vec<String> = (0..count).map(|n| format!("Layer{}", n + 1)).collect();
         for name in &names {
             let mut dl = DocLayer::new(name);