perf(bench): introduce standardized 25-metric CAD and UI benchmark suite

- Add comprehensive performance benchmark harness in benches/performance_benchmarks.rs
  configured with harness = false in Cargo.toml (runs via cargo bench in --release,
  isolated from cargo test).
- Benchmark coverage across 25 distinct CAD & UI operations:
  * Entity Ingestion, GPU Wire Partitioning, Analytical & Wide/Tapered Arc Tessellation
  * 100x Zoom Navigation, Spatial InteractionIndex, Multi-mode Hit-Testing, OSnap Tracking
  * 2D Parametric Dogleg Constraint Solving, DXF Import/Export Throughput
  * UI Ribbon View Construction, Viewport Grid Projection, Grid Cache Evaluation
  * Themed SVG Icon Lookup Caching, Cold Extents AABB Calculation
  * Batch Entity Transformation, Draworder Depth Map Full/Patch, Delta-Undo Recording
- Expose internal calculation hooks with #[doc(hidden)] pub to prevent API leakage.
- Remove 644 lines of legacy, scattered ad-hoc #[test] #[ignore] benchmarks across src/.
- Add developer and regression documentation in benches/README.md.

benches/README.md

@@ -0,0 +1,91 @@
+# OpenCADStudio Performance Benchmarks
+
+A standardized, statistical performance benchmarking suite measuring throughput, latency, and memory metrics across core CAD subsystems and the user interface.
+
+## Quick Start
+
+Run the full benchmark suite with native `--release` optimizations:
+```powershell
+cargo bench
+```
+
+Run a fast validation smoke test (~2 seconds):
+```powershell
+cargo bench --bench performance_benchmarks -- --quick
+```
+
+> [!NOTE]
+> Benchmarks are registered under `[[bench]]` with `harness = false`. They are **never** executed by regular `cargo test` runs, preventing CI slowdowns.
+
+---
+
+## Benchmark Suite Catalog (25 Metrics)
+
+### 1. Scene & Entity Management
+- `scene_entity_ingestion_10k`: Ingestion throughput for 10,000 mixed 2D entities (Lines, Circles, Arcs, Polylines).
+- `batch_entity_transform_mutation`: Translating 1,000 entities, invoking `bump_entities`, and updating the spatial acceleration structure.
+- `undo_delta_recording_cycle`: Capturing before-images and transactional delta recordings for 500 entity mutations.
+
+### 2. Geometry Tessellation & Viewport Navigation
+- `analytical_curved_tessellation`: Initial wire generation for 14,000 curved entities (circles, arcs, ellipses, polylines).
+- `wire_partitioning_per_frame`: Partitioning 14,000 wires into GPU analytical instances vs regular lines.
+- `analytical_curved_zoom_100x`: 100 camera zoom navigations measuring frame overhead and FPS.
+- `wide_and_tapered_arc_tessellation`: Tessellating 7,500 thick polyline arcs, tapered arcs, and donuts.
+- `zoom_extents_bounding_box_calc`: Cold axis-aligned bounding box (AABB) calculation across 20,000 mixed entities (`ZOOM EXTENTS`).
+
+### 3. Spatial Indexing, Snapping & Picking
+- `spatial_interaction_index_build`: Building the spatial `InteractionIndex` and `prepare_screen` for 10,000 wires.
+- `hit_test_click_pick_latency`: Single-point cursor click selection across 5,000 entities in screen space.
+- `hit_test_box_window_enclosed`: Window selection (`box_hit` enclosed) across 5,000 entities.
+- `hit_test_box_crossing_boundary`: Crossing selection (`box_hit` crossing boundary) across 5,000 entities.
+- `osnap_cursor_tracking_latency`: Real-time cursor snapping across Endpoint, Midpoint, Center, Intersection, and Nearest modes.
+
+### 4. Parametrics & DXF I/O
+- `constraint_solver_dogleg_system`: Solving coupled 2D geometric constraints (Perpendicular, Parallel, Length) via the Dogleg solver.
+- `dxf_export_write_throughput`: Serializing 5,000 entities to DXF binary stream.
+- `dxf_import_parse_throughput`: Parsing DXF stream of 5,000 entities into `CadDocument`.
+
+### 5. UI & Viewport Overlay Performance
+- `ui_ribbon_view_construction`: Per-frame ribbon widget tree construction.
+- `ui_grid_geometry_uncached`: Multi-pane tiled viewport grid projection and segment generation.
+- `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.
+
+### 6. State Management & Draworder
+- `selection_state_deep_clone`: Deep cloning of `SelectionState` (with 64 polygon points).
+- `selection_state_arc_clone`: `Arc<SelectionState>::clone` (atomic reference bump).
+- `draworder_depth_map_full_build`: Full draworder depth map generation across 10,000 ordered entities.
+- `draworder_depth_map_incremental_patch`: Fast-path incremental depth map patch on entity modification.
+
+---
+
+## Filtering Benchmarks
+
+Filter execution by subsystem or name pattern:
+```powershell
+# Filter by UI
+cargo bench --bench performance_benchmarks -- ui
+
+# Filter by tessellation
+cargo bench --bench performance_benchmarks -- tessellation
+
+# Filter by hit testing
+cargo bench --bench performance_benchmarks -- hit_test
+```
+
+---
+
+## Baseline Tracking & Regression Detection
+
+### 1. Save Current Metrics as Baseline
+```powershell
+cargo bench --bench performance_benchmarks -- --output target/baseline_metrics.json
+```
+
+### 2. Compare PR / Branch Against Baseline
+```powershell
+cargo bench --bench performance_benchmarks -- --baseline target/baseline_metrics.json
+```
+
+The benchmark runner will output statistical deltas (`% faster` / `% slower`) for each metric and flag regressions exceeding the defined thresholds.