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.

Cargo.toml

@@ -136,3 +136,7 @@ iced = { git = "https://github.com/iced-rs/iced.git", rev = "23604ff22ab0aad9e00
 
 [profile.release]
 strip = true
+
+[[bench]]
+name = "performance_benchmarks"
+harness = false