perf: build hatch/image/mesh caches off the UI thread

Parsing + DerivedCaches construction now runs entirely in a dedicated
OS thread inside open_path(). FileOpened handler only assigns the
pre-built caches, keeping the UI thread free during file load.

- Add build_derived_caches(): parallel hatch + mesh tessellation via rayon
- Extend Message::FileOpened / open_path return type with DerivedCaches
- Drop populate_*_from_document() calls from FileOpened handler

src/app/mod.rs

@@ -212,7 +212,7 @@ pub enum DsField {
 pub enum Message {
     Tick(Instant),
     OpenFile,
-    FileOpened(Result<(String, PathBuf, CadDocument), String>),
+    FileOpened(Result<(String, PathBuf, CadDocument, crate::scene::DerivedCaches), String>),
     SaveFile,
     SaveAs,
     // ── Custom Save-As dialog ─────────────────────────────────────────────