IGES 5.3 import and export; OBJ import; STL reading hardened

- IGES import (kernel iges_in, from the published specification). The file
  is restated as STEP and read by the STEP reader, so an IGES file becomes
  the same Import base feature.
  - Manifold solid B-reps (186/514/510/508/504/502).
  - Trimmed (144), bounded (143) and bare B-spline surfaces, sewn into a
    solid by shared edges, with face orientation made consistent.
  - Curves: lines, arcs, ellipses, composite and rational B-spline curves.
  - Surfaces: planes, analytic surfaces 190-198, B-spline surfaces,
    surfaces of revolution and tabulated cylinders.
  - Transformations, units, colours, names (406 form 15 or labels).
  Open, insert and drop .igs/.iges like STEP.
- IGES export (kernel iges_out): each body as a manifold solid B-rep on the
  surfaces a STEP export uses, with name properties and colours.
  ExportCommand to .igs.
- OBJ import as mesh bodies: objects/groups, polygons fanned, negative
  indices, bad lines reported.
- STL: binary files whose header starts with "solid" and those with a zero
  triangle count read as binary; non-finite vertices dropped; facet
  normals ignored; ASCII with non-UTF-8 names. Mesh bodies get consistent
  winding (stray inverted facets turned over).
- Tests: primitives and features round-trip through IGES (volume, area,
  name, colour). Six trimmed patches sew into a closed box. Hostile IGES,
  OBJ and STL inputs never panic. STL quirks. The engine exports a design
  to IGES and opens it again. All 76 oracle parts round-trip through IGES
  with matching volume (local check).

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01AS5Tn3xieewUSg3uxrMVdL

README.md

@@ -30,12 +30,14 @@ storytold "craft" family.
   deleted.
 - **Parameters** with unit-aware expressions (`2 * width + 5 mm`, `angle / 2`).
 - **Files**: designs as JSON (`.solvecraft`), export to STEP (AP242 with body names, colours and
-  components as assemblies; revolved faces as native planes, cylinders, cones, spheres and tori), 3MF, STL (binary/ASCII) and OBJ;
-  3MF and STL import as mesh bodies (they render, measure, move and export; solid features need
-  B-rep bodies);
-  STEP import (AP203/AP214/AP242 solids, assemblies, units, names, colours) as a base feature
-  that later features build on — open a `.step`/`.stp` file, insert one into a design, or drop it
-  on the window ([docs/step-import.md](docs/step-import.md)).
+  components as assemblies; revolved faces as native planes, cylinders, cones, spheres and tori),
+  IGES 5.3 (manifold solid B-reps with names and colours), 3MF, STL (binary/ASCII) and OBJ;
+  3MF, STL and OBJ import as mesh bodies (they render, measure, move and export; solid features
+  need B-rep bodies);
+  STEP import (AP203/AP214/AP242 solids, assemblies, units, names, colours) and IGES import
+  (manifold solid B-reps, or trimmed and bounded surfaces sewn into solids) as a base feature
+  that later features build on — open a `.step`/`.stp`/`.igs`/`.iges` file, insert one into a
+  design, or drop it on the window ([docs/step-import.md](docs/step-import.md)).
 - **Desktop app** (egui + wgpu): toolbar with workspace tabs, browser, timeline, 3D viewport with
   view cube and navigation bar, sketch tools, feature dialogs, command palette, and a JSON-lines
   control channel so agents and tests can drive it.