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/scene/render.rs

@@ -39,7 +39,7 @@ pub struct ViewportData {
     /// by the wire / face3d pipelines as a clip-z bias. WireModels carry no
     /// depth field (84 construction sites); the bias is looked up by handle
     /// at GPU-upload time from this map instead.
-    pub(super) draw_depths: Arc<std::collections::HashMap<u64, f32>>,
+    pub(super) draw_depths: Arc<rustc_hash::FxHashMap<u64, f32>>,
     pub(super) hatches: Arc<Vec<HatchModel>>,
     /// Wipeout fills — rendered in a separate pass AFTER wires.
     pub(super) wipeout_hatches: Arc<Vec<HatchModel>>,