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/helpers.rs

@@ -171,7 +171,7 @@ pub(super) fn entities_centroid(wires: &[WireModel]) -> glam::Vec3 {
 
 /// Generate the next available auto group name ("*A1", "*A2", …).
 pub(super) fn next_group_auto_name(scene: &crate::scene::Scene) -> String {
-    let existing: std::collections::HashSet<String> =
+    let existing: rustc_hash::FxHashSet<String> =
         scene.groups().map(|g| g.name.clone()).collect();
     for n in 1..=9999 {
         let name = format!("*A{n}");