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

xtask/src/main.rs

@@ -249,6 +249,16 @@ solid's name, else the product's); surface colours; assemblies (next-assembly-us
 transformations, and mapped items) — the product tree is kept in the feature for the components milestone. Files are \
 untrusted: sizes, entity counts and nesting are capped and every kernel call is guarded. What cannot be read becomes a \
 warning on the feature (a face left out gives an open body); not read yet: offset surfaces, pcurve-only edges.\n\n\
+## IGES\n\n\
+IGES 5.3 files (`.igs`/`.iges`) open, insert and drop the same way: `crates/kernel/src/iges_in.rs` (written from the \
+published specification) restates the file as STEP and the STEP reader reads it, so the feature holds STEP text. Read: \
+manifold solid B-reps (186) with their shells, faces, loops, edge and vertex lists; trimmed (144) and bounded (143) \
+surfaces, and bare B-spline patches, sewn into a solid by their shared edges (an open body when they do not close); \
+lines, circular arcs, ellipses, composite curves, rational B-spline curves; planes (108, 190), cylinders, cones, \
+spheres, tori, rational B-spline surfaces, surfaces of revolution and tabulated cylinders; transformation matrices; \
+units; colours (314 and the predefined numbers); body names (name properties, else labels). Not read yet: ruled and \
+offset surfaces, curves on surfaces given only in parameter space. Export (`ExportCommand` to `.igs`) writes each body as \
+a manifold solid B-rep on the same surfaces a STEP export uses, with its name and colour.\n\n\
 ## Fusion corpus\n\nGenerated by `cargo xtask step-corpus`. Each case is a part exported from Fusion as STEP (AP214; kept locally in `plan/fusion/oracle/`, never committed). SolveCraft opens `part.step` exactly as File → Open does (an Import base feature) and compares with Fusion's own measurements: body count, volume and area (relative tolerance 1e-3) and the number of faces read from the file.\n\n**{pass} / {} files pass.**\n\n| Case | Result | Bodies | Volume mm³ (got / Fusion) | Area mm² (got / Fusion) | Faces (got / Fusion) | ms | Notes |\n|---|---|---|---|---|---|---|---|\n",
         list.len()
     );