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,
         }