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/tess.rs

@@ -665,7 +665,7 @@ pub(crate) fn tessellate_entity(
     // linetype assigned, so we only consult the first wire here — multi-wire
     // returns come exclusively from MTEXT colour splits which can't trigger
     // this path.
-    if let Some(clt) = crate::linetypes::complex_lt(lt_name) {
+    if let Some(clt) = crate::io::linetypes::complex_lt(lt_name) {
         if let Some(base) = bases.first() {
             let mut wires = text::complex_lt::apply_along(
                 &base.name,