chore(perf): profile frame pipeline CPU side

Adds ui_selection_overlay bench (benches/performance_benchmarks.rs + README metric 31): builds a representative selection overlay element (snap, crosshair, UCS tripod, 128 grips quick / 4096 capped full) and times widget-tree construction. Measured 0.4us typical, 2.4us at the cap (threshold 50us).

Adds ribbon/statusbar section marks to the chrome bucket in view_main (src/app/view/mod.rs) so live PERF=1 view-detail traces attribute those two costs separately. Gated on the existing perf switch; no-op otherwise.

Adds pub re-exports of CursorType/IsoPlane (src/app/mod.rs) for the bench fixture (same pattern as the existing grip re-exports).

Ranked result over all ui_* benches: ribbon view construction ~556us (largest; follow-up already tracked), everything else under 50us or synthetic-fixture shapes. No new follow-up filed. GPU and canvas-draw costs are unmeasurable in headless runs (view() never executes there).

benches/README.md

@@ -19,7 +19,7 @@ cargo bench --bench performance_benchmarks -- --quick
 
 ---
 
-## Benchmark Suite Catalog (30 Metrics)
+## Benchmark Suite Catalog (31 Metrics)
 
 ### 1. Scene & Entity Management
 - `scene_entity_ingestion_10k`: Ingestion throughput for 10,000 mixed 2D entities (Lines, Circles, Arcs, Polylines).
@@ -56,6 +56,7 @@ cargo bench --bench performance_benchmarks -- --quick
 - `ui_grip_budget`: Budgeted per-frame selection-grip projection (dense-polyline grips capped to `MAX_SELECTED_GRIPS`).
 - `ui_grip_budget_build`: Selection-change `apply_grip_budget` cost over the full grip set (sort + FxHashSet + filter).
 - `ui_constraint_glyphs`: Constraint-glyph cache hit (`cached_glyph_placements` key build + `Arc` clone over dozens of glyphs, no recompute).
+- `ui_selection_overlay`: Selection overlay Element construction (widget tree with capped grip count, no canvas draw).
 
 ### 6. State Management & Draworder
 - `selection_state_deep_clone`: Deep cloning of `SelectionState` (with 64 polygon points).