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/scene/model/hatch_patterns.rs

@@ -6,7 +6,7 @@
 
 use std::sync::OnceLock;
 
-use crate::patterns::PatLineDef;
+use crate::io::patterns::PatLineDef;
 use crate::scene::model::hatch_model::{HatchPattern, PatFamily};
 use acadrust::entities::{HatchPattern as DxfPattern, HatchPatternLine};
 use acadrust::types::Vector2;
@@ -65,7 +65,7 @@ fn build_catalog() -> Vec<PatternEntry> {
         pat_lines: vec![],
     }];
 
-    for def in crate::patterns::catalog() {
+    for def in crate::io::patterns::catalog() {
         entries.push(PatternEntry {
             name: def.name.clone(),
             description: def.description.clone(),