perf(statusbar): cache layout/scale derived data behind epochs (major)

Status bar rescanned the whole object map twice per frame (layout_names +
scale_picker_list), deep-cloned an already-cached Arc, and built two
undo/redo label vecs consumed ~never. Cache layout names (epoch key) and
scale picker (epoch + unit-factor + imperial-family + current-scale key)
as Arc behind choke-point invalidation (layout/scale impl methods,
undo/redo swaps, file-open, doc-reset); borrow across the widget boundary
(owned-Arc params; E0515 forbids &'a slices over call-site temporaries);
build undo/redo labels only when their dropdown is open.

Tests:
- cached_layout_names_returns_same_arc_until_bump: hit-path Arc identity
- cached_layout_names_invalidated_by_add_layout: bump yields new Arc + names
- cached_scale_picker_returns_same_arc_until_bump: hit-path Arc identity
- cached_scale_picker_invalidated_by_add_scale: bump yields new Arc + scale
- cached_scale_picker_invalidated_by_current_scale_change: key completeness

Bench (cargo bench ui_statusbar_derived_data, 3k entities): cached hit below
timer granularity vs ~1us uncached scans, PASS (50us threshold).

benches/README.md

@@ -19,7 +19,7 @@ cargo bench --bench performance_benchmarks -- --quick
 
 ---
 
-## Benchmark Suite Catalog (26 Metrics)
+## Benchmark Suite Catalog (27 Metrics)
 
 ### 1. Scene & Entity Management
 - `scene_entity_ingestion_10k`: Ingestion throughput for 10,000 mixed 2D entities (Lines, Circles, Arcs, Polylines).
@@ -52,6 +52,7 @@ cargo bench --bench performance_benchmarks -- --quick
 - `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).
+- `ui_statusbar_derived_data`: Status-bar cached derived-data hit (layout names + scale picker `Arc` clones).
 
 ### 6. State Management & Draworder
 - `selection_state_deep_clone`: Deep cloning of `SelectionState` (with 64 polygon points).