perf(hash): swap std HashMap/HashSet for rustc-hash FxHash

Phase 4.1. Handle / u64 keys dominate the hot maps (block_defn,
hatch / image / mesh caches, draw-depth map, viewport wire caches);
the default SipHash is wasted work on integer keys. Alias the std
types to rustc_hash::FxHashMap / FxHashSet crate-wide so every map and
set — including the cross-module draw-depth map shared between scene
and the GPU pipelines — uses the faster hasher consistently.

Mechanical: no behavioural change. Constructors moved from ::new() to
::default() (Fx variants carry no RandomState). Build + 13 tests green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/update.rs

@@ -214,7 +214,7 @@ impl OpenCADStudio {
                 self.tabs[i].scene.meshes = caches.meshes;
                 // Invalidate the wire cache so the new document is tessellated.
                 self.tabs[i].scene.bump_geometry();
-                self.tabs[i].scene.selected = std::collections::HashSet::new();
+                self.tabs[i].scene.selected = rustc_hash::FxHashSet::default();
                 self.tabs[i].scene.preview_wires = vec![];
                 self.tabs[i].scene.current_layout = "Model".to_string();
                 crate::linetypes::populate_document(&mut self.tabs[i].scene.document);