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/ui/ribbon/mod.rs

@@ -6,7 +6,7 @@
 //
 // Dropdown items within a group are collected into columns of 3 rows.
 
-use std::collections::HashMap;
+use rustc_hash::FxHashMap as HashMap;
 
 use acadrust::types::{Color as AcadColor, LineWeight};
 use iced::widget::{button, column, container, mouse_area, row, scrollable, svg, text};
@@ -73,7 +73,7 @@ impl Ribbon {
             wireframe: false,
             ortho_mode: true,
             open_dropdown: None,
-            last_cmd: HashMap::new(),
+            last_cmd: HashMap::default(),
             layer_names: vec!["0".to_string()],
             active_layer: "0".to_string(),
             layer_infos: vec![LayerInfo {

src/ui/ribbon/widgets.rs

@@ -1,7 +1,7 @@
 // Shared rendering helpers, button styles, colours, layout constants, and
 // free functions used by the Ribbon view/overlay methods.
 
-use std::collections::HashMap;
+use rustc_hash::FxHashMap as HashMap;
 use std::time::Duration;
 
 use acadrust::types::{Color as AcadColor, LineWeight};