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/block_cache.rs

@@ -15,7 +15,7 @@
 // a visited set so a self-referential block produces a marker rather than
 // recursing forever.
 
-use std::collections::HashMap;
+use rustc_hash::FxHashMap as HashMap;
 use std::sync::Arc;
 
 use acadrust::types::{Color as AcadColor, LineWeight, Transform, Vector3};
@@ -206,8 +206,8 @@ impl BlockCache {
 /// Walk all entities + all block_record contents collecting every distinct
 /// `block_name` that appears in an Insert (transitively).
 fn collect_referenced_blocks(doc: &CadDocument) -> Vec<String> {
-    use std::collections::HashSet;
-    let mut seen: HashSet<String> = HashSet::new();
+    use rustc_hash::FxHashSet as HashSet;
+    let mut seen: HashSet<String> = HashSet::default();
     let mut queue: Vec<String> = Vec::new();
 
     for entity in doc.entities() {