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

include/vde/foundation/format_utils.h

@@ -0,0 +1,59 @@
+#pragma once
+#include "vde/core/point.h"
+#include <string>
+#include <sstream>
+#include <iomanip>
+#include <cmath>
+#include <cstdio>
+
+namespace vde::foundation {
+
+/// Format double with adequate precision for CAD (12 significant digits)
+inline std::string fmt_double(double v, int precision = 12) {
+    if (std::isnan(v)) return "0.0";
+    if (std::isinf(v)) return v > 0 ? "1e30" : "-1e30";
+    std::ostringstream ss;
+    ss << std::setprecision(precision) << std::scientific << v;
+    std::string s = ss.str();
+    // Ensure always has exponent sign (e.g., "1.0e+00" not "1.0e00")
+    auto epos = s.find('e');
+    if (epos != std::string::npos) {
+        if (epos + 1 < s.size() && s[epos + 1] != '-' && s[epos + 1] != '+') {
+            s.insert(epos + 1, "+");
+        }
+    }
+    return s;
+}
+
+/// Format for IGES: uses D exponent
+inline std::string fmt_iges_real(double v) {
+    auto s = fmt_double(v, 12);
+    auto pos = s.find('e');
+    if (pos != std::string::npos) s[pos] = 'D';
+    return s;
+}
+
+/// Format a Point3D as comma-separated reals
+inline std::string fmt_point(const core::Point3D& p, bool use_d = false) {
+    auto fmt = use_d ? fmt_iges_real : fmt_double;
+    return fmt(p.x()) + "," + fmt(p.y()) + "," + fmt(p.z());
+}
+
+/// Pad a string to exactly 72 characters with spaces
+inline std::string pad72(const std::string& s) {
+    std::string result = s;
+    if (result.size() > 72) result.resize(72);
+    result.resize(72, ' ');
+    return result;
+}
+
+/// Format an IGES section line: data + section marker (80 columns total)
+inline std::string format_iges_line(const std::string& data, char section_char, int seq) {
+    std::string line = pad72(data);
+    char seq_buf[9];
+    std::snprintf(seq_buf, sizeof(seq_buf), "%c%07d", section_char, seq);
+    line += std::string(seq_buf);
+    return line + "\n";
+}
+
+} // namespace vde::foundation