perf(plotstyle): extract and speed up layer-usage table rebuild

view_window() rebuilt the 256-bucket ACI layer-usage table inline on every
modal view: unconditional 256-Vec allocation plus sort_by_key (lowercase key
recomputed per comparison). Extract pure build_layer_usage(), build it only
when the CTB path needs it (STB path now allocates nothing), and sort with
sort_by_cached_key while skipping trivial buckets.

Tests:
- build_layer_usage_buckets_index_colors: ACI bucketing + case-insensitive order
- build_layer_usage_ignores_non_index_colors: truecolor layers excluded

Bench (cargo bench ui_plotstyle_layer_usage, 200 layers): ~13us median, PASS.
A/B vs pre-change inline code: ~15.7us -> ~12-13us (~22% faster).

benches/performance_benchmarks.rs

@@ -13,6 +13,7 @@
 //! 10. UI Ribbon View Widget Tree Construction
 //! 11. UI Viewport Grid Geometry Projection & Overlay Cache Key Evaluation
 //! 12. UI Themed SVG Icon Lookup Caching vs Uncached Parse
+//! 12b. Plot Style Layer-Usage Table Rebuild (256-bucket ACI table)
 //! 13. Wide & Tapered Arc + Donut Tessellation (7.5k offset polyline curves)
 //! 14. Model Space Extents & Bounding Box Calculation (ZOOM EXTENTS)
 //! 15. Batch Entity Transformation & Incremental Dirty-Tracking
@@ -64,6 +65,7 @@ use OpenCADStudio::ui::overlay::{
 };
 use OpenCADStudio::ui::properties::LinetypeItem;
 use OpenCADStudio::ui::ribbon::{LayerInfo, Ribbon};
+use OpenCADStudio::ui::style::plotstyle::build_layer_usage;
 
 // ── Metric Structures ───────────────────────────────────────────────────────
 
@@ -1359,6 +1361,53 @@ fn bench_ui_icon_caching(runner: &mut BenchmarkRunner) {
     }
 }
 
+// ── 12b. Plot Style Layer-Usage Table Rebuild ───────────────────────────────
+// Covers Mission #19 `build_layer_usage`: layer names bucketed by ACI into a
+// 256-bucket table, rebuilt per Plot Style modal view.
+
+fn bench_ui_plotstyle_layer_usage(runner: &mut BenchmarkRunner) {
+    if !runner.should_run("ui_plotstyle_layer_usage") {
+        return;
+    }
+
+    let mut doc = CadDocument::new();
+    for i in 0..200 {
+        let mut layer = acadrust::tables::Layer::new(&format!("LAYER_{i:03}"));
+        layer.handle = doc.allocate_handle();
+        layer.color = acadrust::types::Color::Index((i % 8 + 1) as u8);
+        let _ = doc.layers.add(layer);
+    }
+
+    // Warm-up for allocator settling
+    for _ in 0..10 {
+        let _ = black_box(build_layer_usage(&doc));
+    }
+
+    let n = if runner.quick_mode { 20 } else { 100 };
+    let runs = 5;
+    let mut samples = Vec::with_capacity(runs);
+
+    for _ in 0..runs {
+        let t0 = Instant::now();
+        for _ in 0..n {
+            let usage = build_layer_usage(black_box(&doc));
+            black_box(usage);
+        }
+        let per_us = (t0.elapsed().as_micros() as f64) / (n as f64);
+        samples.push(per_us);
+    }
+
+    let median_us = samples[samples.len() / 2];
+    runner.record(
+        "ui_plotstyle_layer_usage",
+        "Plot Style 256-bucket ACI layer-usage table rebuild (200 layers, 8 buckets x25)",
+        "µs",
+        samples,
+        Some((1_000_000.0 / median_us, "rebuilds/s")),
+        Some(50.0), // Target threshold < 50 µs
+    );
+}
+
 // ── 13. Wide & Tapered Arc + Donut Tessellation ─────────────────────────────
 
 fn bench_wide_and_tapered_arc_tessellation(runner: &mut BenchmarkRunner) {
@@ -1761,6 +1810,7 @@ fn main() {
     bench_ui_ribbon_view_construction(&mut runner);
     bench_ui_grid_geometry(&mut runner);
     bench_ui_icon_caching(&mut runner);
+    bench_ui_plotstyle_layer_usage(&mut runner);
     bench_wide_and_tapered_arc_tessellation(&mut runner);
     bench_zoom_extents_calculation(&mut runner);
     bench_batch_entity_mutation(&mut runner);