feat(brep): S13-B IGES export + format utilities

Add IGES v5.3 export for B-Rep models with:
- Entity mapping: Point(116), Line(110), Circular Arc(100),
  B-Spline Curve(126), Plane(108), B-Spline Surface(128),
  Vertex(502), Edge(504), Loop(508), Face(510), Shell(514),
  Manifold Solid B-Rep(186)
- Auto-detection of circular arcs (degree-2 NURBS with
  isosceles control triangle) and planes (1x1 degree surfaces)
- Fixed 80-column IGES format with S/G/D/P/T sections
- Header-only format_utils.h for IGES/STEP number formatting

New files:
- include/vde/foundation/format_utils.h
- include/vde/brep/iges_export.h
- src/brep/iges_export.cpp
- tests/brep/test_iges_export.cpp

include/vde/brep/iges_export.h

@@ -0,0 +1,27 @@
+#pragma once
+#include "vde/brep/brep.h"
+#include <string>
+#include <vector>
+
+namespace vde::brep {
+
+/// Export B-Rep bodies to IGES format string (version 5.3)
+/// Supports common entity types for maximum compatibility:
+///   - Type 116: Point
+///   - Type 110: Line
+///   - Type 100: Circular Arc
+///   - Type 126: Rational B-Spline Curve
+///   - Type 108: Plane
+///   - Type 128: Rational B-Spline Surface
+///   - Type 502: Vertex (B-Rep topology)
+///   - Type 504: Edge
+///   - Type 508: Loop
+///   - Type 510: Face
+///   - Type 514: Shell
+///   - Type 186: Manifold Solid B-Rep Object
+[[nodiscard]] std::string export_iges(const std::vector<BrepModel>& bodies);
+
+/// Export to IGES file
+void export_iges_file(const std::string& filepath, const std::vector<BrepModel>& bodies);
+
+} // namespace vde::brep