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 <[email protected]>

src/main.rs

@@ -10,16 +10,13 @@ mod cli;
 mod command;
 mod entities;
 mod io;
-mod linetypes;
 mod modules;
 mod plugin;
-mod patterns;
 mod scene;
 mod snap;
 mod ui;
 mod par;
 mod sys;
-mod update_check;
 
 fn main() -> iced::Result {
     // Web (wasm) uses the single-window entry; native uses the multi-window