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/properties.rs

@@ -7,7 +7,7 @@
 //! • Linetype   → read-only for now
 //! • Geometry   → text_input per coordinate / dimension field
 
-use std::collections::HashMap;
+use rustc_hash::FxHashMap as HashMap;
 use std::fmt;
 
 use crate::ui::ROW_H;
@@ -142,12 +142,12 @@ impl Default for PropertiesPanel {
             selected_group: None,
             linetype_items: vec![],
             selection_group_combo: combo_box::State::new(vec![]),
-            choice_combos: HashMap::new(),
+            choice_combos: HashMap::default(),
             layer_combo: combo_box::State::new(vec![]),
             lineweight_combo: combo_box::State::new(lw_options()),
             linetype_combo: combo_box::State::new(vec![]),
             hatch_pattern_combo: combo_box::State::new(crate::scene::hatch_patterns::names()),
-            edit_buf: HashMap::new(),
+            edit_buf: HashMap::default(),
             color_picker_open: false,
             color_palette_open: false,
         }

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};

src/ui/selection_filter_popup.rs

@@ -2,7 +2,7 @@
 //! A checked row means that type can be picked; unchecking it excludes the
 //! type from interactive selection. Opened from the FILTER status pill.
 
-use std::collections::HashSet;
+use rustc_hash::FxHashSet as HashSet;
 
 use iced::widget::{button, column, container, mouse_area, row, text};
 use iced::{Background, Border, Color, Element, Fill, Length, Padding, Theme};

src/ui/shortcuts.rs

@@ -79,7 +79,7 @@ fn section<'a>(title: &'static str) -> Element<'a, Message> {
 }
 
 pub fn view_window<'a>(
-    overrides: &'a std::collections::HashMap<String, String>,
+    overrides: &'a rustc_hash::FxHashMap<String, String>,
 ) -> Element<'a, Message> {
     // ── Toolbar ───────────────────────────────────────────────────────────
     let toolbar = container(

src/ui/statusbar_config.rs

@@ -5,7 +5,7 @@
 //! adds or removes that pill from the bar. The choice is persisted so it
 //! survives across sessions.
 
-use std::collections::HashSet;
+use rustc_hash::FxHashSet as HashSet;
 use std::path::PathBuf;
 
 /// Identifies a toggleable status-bar pill.
@@ -121,7 +121,7 @@ pub struct StatusBarConfig {
 impl StatusBarConfig {
     /// Load the saved customization, or all-visible when none exists.
     pub fn load() -> Self {
-        let mut hidden = HashSet::new();
+        let mut hidden = HashSet::default();
         if let Some(path) = config_path() {
             if let Ok(body) = std::fs::read_to_string(path) {
                 for line in body.lines() {