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>

ROADMAP.md

@@ -203,7 +203,7 @@ matters.
 
 ## Phase 4 — Allocation & Memory
 
-### 4.1 Swap `HashMap` for `rustc-hash::FxHashMap`
+### 4.1 Swap `HashMap` for `rustc-hash::FxHashMap` ✅ DONE
 
 `Handle` is an integer wrapper; the default `SipHash` is overkill.
 `FxHashMap` gives 20-40 % in hash-heavy sites (block_cache, hatches /