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>

Cargo.toml

@@ -28,6 +28,9 @@ rayon = "1"
 ureq = { version = "3", default-features = false, features = ["rustls"] }
 inventory = "0.3"
 truck-shapeops = "0.4"
+# Fast non-cryptographic hasher. Handle/u64 keys dominate the hot block,
+# hatch, draw-depth and wire caches; the default SipHash is overkill there.
+rustc-hash = "2"
 
 [target.'cfg(target_os = "windows")'.dependencies]
 windows-sys = { version = "0.61", features = ["Win32_UI_Shell", "Win32_UI_WindowsAndMessaging"] }