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/README.md

@@ -19,7 +19,7 @@ cargo bench --bench performance_benchmarks -- --quick
 
 ---
 
-## Benchmark Suite Catalog (25 Metrics)
+## Benchmark Suite Catalog (26 Metrics)
 
 ### 1. Scene & Entity Management
 - `scene_entity_ingestion_10k`: Ingestion throughput for 10,000 mixed 2D entities (Lines, Circles, Arcs, Polylines).
@@ -51,6 +51,7 @@ cargo bench --bench performance_benchmarks -- --quick
 - `ui_grid_cache_hit_evaluation`: Grid overlay cache key construction and reuse decision.
 - `ui_icon_handle_cached`: Themed SVG icon cache hit (FxHashMap lookup + Handle clone).
 - `ui_icon_handle_uncached`: Uncached SVG icon memory parse and Handle allocation.
+- `ui_plotstyle_layer_usage`: Plot Style 256-bucket ACI layer-usage table rebuild (200 layers).
 
 ### 6. State Management & Draworder
 - `selection_state_deep_clone`: Deep cloning of `SelectionState` (with 64 polygon points).

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);

src/ui/style/plotstyle.rs

@@ -84,6 +84,35 @@ fn field_style(theme: &Theme, status: text_input::Status) -> text_input::Style {
     }
 }
 
+/// Layer names bucketed by ACI (AutoCAD Color Index), index = ACI.
+///
+/// Pure function of `document.layers`: one entry per layer whose color is
+/// `Color::Index(aci)` with `1 <= aci < 256`, each bucket sorted
+/// case-insensitively. Extracted from `view_window` so the table can be
+/// unit-tested and benched without building widgets. `pub` (not
+/// `pub(crate)`) so the `cargo bench` harness (`benches/`, an external
+/// crate) can measure it as `ui_plotstyle_layer_usage`.
+pub fn build_layer_usage(document: &acadrust::CadDocument) -> Vec<Vec<String>> {
+    let mut layer_usage = vec![Vec::<String>::new(); 256];
+
+    for layer in document.layers.iter() {
+        if let acadrust::types::Color::Index(aci) = &layer.color {
+            let index = *aci as usize;
+
+            if index > 0 && index < layer_usage.len() {
+                layer_usage[index].push(layer.name.clone());
+            }
+        }
+    }
+
+    for layers in &mut layer_usage {
+        if layers.len() > 1 {
+            layers.sort_by_cached_key(|name| name.to_ascii_lowercase());
+        }
+    }
+    layer_usage
+}
+
 pub fn view_window<'a>(
     document: &'a acadrust::CadDocument,
     table: Option<&'a crate::io::plot_style::PlotStyleTable>,
@@ -95,23 +124,11 @@ pub fn view_window<'a>(
 ) -> Element<'a, Message> {
     let show_layer_usage = !table.is_some_and(|table| table.is_stb);
 
-let mut layer_usage = vec![Vec::<String>::new(); 256];
-
-    if show_layer_usage {
-        for layer in document.layers.iter() {
-            if let acadrust::types::Color::Index(aci) = &layer.color {
-                let index = *aci as usize;
-
-                if index > 0 && index < layer_usage.len() {
-                    layer_usage[index].push(layer.name.clone());
-                }
-            }
-        }
-
-        for layers in &mut layer_usage {
-            layers.sort_by_key(|name| name.to_ascii_lowercase());
-        }
-    }
+    let layer_usage = if show_layer_usage {
+        build_layer_usage(document)
+    } else {
+        Vec::new()
+    };
     let table_name = table
         .map(|t| t.name.clone())
         .unwrap_or_else(|| t!("(no table loaded)").into_owned());
@@ -536,3 +553,58 @@ let mut layer_usage = vec![Vec::<String>::new(); 256];
         .height(sizing.height)
         .into()
 }
+
+#[cfg(test)]
+mod layer_usage_tests {
+    use super::*;
+
+    fn indexed_layer(
+        doc: &mut acadrust::CadDocument,
+        name: &str,
+        aci: u8,
+    ) -> acadrust::tables::Layer {
+        let mut layer = acadrust::tables::Layer::new(name);
+        layer.handle = doc.allocate_handle();
+        layer.color = acadrust::types::Color::Index(aci);
+        layer
+    }
+
+    #[test]
+    fn build_layer_usage_buckets_index_colors() {
+        let mut doc = acadrust::CadDocument::new();
+        let a = indexed_layer(&mut doc, "WALLS", 1);
+        let b = indexed_layer(&mut doc, "doors", 1);
+        let c = indexed_layer(&mut doc, "ROOF", 3);
+        let _ = doc.layers.add(a);
+        let _ = doc.layers.add(b);
+        let _ = doc.layers.add(c);
+
+        let usage = build_layer_usage(&doc);
+
+        assert_eq!(usage.len(), 256);
+        assert_eq!(usage[1], vec!["doors".to_string(), "WALLS".to_string()]);
+        assert_eq!(usage[3], vec!["ROOF".to_string()]);
+        assert!(usage[2].is_empty());
+    }
+
+    #[test]
+    fn build_layer_usage_ignores_non_index_colors() {
+        let mut doc = acadrust::CadDocument::new();
+        let mut truecolor = acadrust::tables::Layer::new("TRUECOLOR");
+        truecolor.handle = doc.allocate_handle();
+        truecolor.color = acadrust::types::Color::Rgb {
+            r: 10,
+            g: 20,
+            b: 30,
+        };
+        let _ = doc.layers.add(truecolor);
+
+        let usage = build_layer_usage(&doc);
+
+        assert!(
+            usage.iter().all(|bucket| !bucket.iter().any(|n| n == "TRUECOLOR")),
+            "truecolor layer must not appear in any ACI bucket"
+        );
+    }
+
+}