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/ui/shortcuts.rs

@@ -79,7 +79,7 @@ fn section<'a>(title: &'static str) -> Element<'a, Message> {
 }
 
 pub fn view_window<'a>(
-    overrides: &'a std::collections::HashMap<String, String>,
+    overrides: &'a rustc_hash::FxHashMap<String, String>,
 ) -> Element<'a, Message> {
     // ── Toolbar ───────────────────────────────────────────────────────────
     let toolbar = container(