feat(brep): S13-A — IGES import support

Implement IGES (Initial Graphics Exchange Specification) file reader that
converts IGES entities to B-Rep models.

Key features:
- 80-character fixed-width card image parser (S/G/D/P/T sections)
- Directory Entry parsing with all 18 DE fields per entity
- Parameter Data parsing with DE pointer tracking
- D exponent notation support (e.g., 1.5D+2 = 150)

Supported IGES entity types:
  Curve entities (→ NurbsCurve):
    Type 100 - Circular Arc (rational degree-2)
    Type 102 - Composite Curve
    Type 104 - Conic Arc (ellipse/hyperbola/parabola)
    Type 106 - Copious Data (polyline)
    Type 110 - Line (degree-1)
    Type 126 - Rational B-Spline Curve

  Surface entities (→ NurbsSurface):
    Type 108 - Plane
    Type 114 - Parametric Spline Surface
    Type 118 - Ruled Surface
    Type 120 - Surface of Revolution
    Type 122 - Tabulated Cylinder (extrude)
    Type 128 - Rational B-Spline Surface
    Type 140 - Offset Surface
    Type 192 - Right Circular Cylindrical Surface
    Type 194 - Right Circular Conical Surface
    Type 196 - Spherical Surface
    Type 198 - Toroidal Surface

  Topology entities (→ BrepModel):
    Type 186 - Manifold Solid B-Rep Object
    Type 502 - Vertex
    Type 504 - Edge
    Type 508 - Loop
    Type 510 - Face
    Type 514 - Shell

Tests cover: line, arc, cylinder, sphere, cone, torus, plane,
B-spline curve/surface, ruled surface, revolution surface,
tabulated cylinder, copious data polyline, manifold solid B-Rep,
D exponent notation, empty/truncated file handling, and file errors.

include/vde/brep/iges_import.h

@@ -0,0 +1,33 @@
+#pragma once
+#include "vde/brep/brep.h"
+#include <string>
+#include <vector>
+
+namespace vde::brep {
+
+/// IGES import error codes
+enum class IgesError {
+    Ok = 0,
+    FileNotFound,
+    ParseError,
+    InvalidSection,
+    UnsupportedEntity,
+    MissingEntity,
+    EntityTypeError,
+};
+
+/// Parse an IGES file (.igs/.iges) and return B-Rep bodies
+/// IGES format: 80-character fixed-width records (ANSI Y14.26M)
+/// Supports the most common entity types found in CAD files
+[[nodiscard]] std::vector<BrepModel> import_iges(const std::string& filepath);
+
+/// Parse IGES data from a string (for testing)
+[[nodiscard]] std::vector<BrepModel> import_iges_from_string(const std::string& data);
+
+/// Get the last error from import_iges / import_iges_from_string
+[[nodiscard]] IgesError iges_last_error();
+
+/// Get a human-readable description of the last error
+[[nodiscard]] const std::string& iges_last_error_message();
+
+} // namespace vde::brep