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/mod.rs

@@ -378,7 +378,7 @@ pub(super) struct OpenCADStudio {
 
     // ── Keyboard Shortcut Editor ──────────────────────────────────────────
     /// User-defined function-key overrides: "F3" → command string.
-    shortcut_overrides: std::collections::HashMap<String, String>,
+    shortcut_overrides: rustc_hash::FxHashMap<String, String>,
 
     // ── Layout Manager Panel ──────────────────────────────────────────────
     layout_manager_selected: String,
@@ -1324,7 +1324,7 @@ impl OpenCADStudio {
             // Color scheme (default: dark CAD-style)
             active_theme: Theme::Dark,
             // Keyboard shortcuts
-            shortcut_overrides: std::collections::HashMap::new(),
+            shortcut_overrides: rustc_hash::FxHashMap::default(),
             // Layout Manager
             layout_manager_selected: "Model".to_string(),
             layout_manager_rename_buf: String::new(),