Add unit tests for the delivery/IO modules

[devin-ai-integration]

Summary

weft_core's delivery layer was effectively untested: the only test binaries exercised meshing (weft_tests) and the signature CLI, so per-file gcov on core/ showed the exporters and the IO registry at or near zero:

  0.0%   0/272   core/src/export_gltf.cpp
  0.0%   0/196   core/src/export_fbx.cpp
  0.0%   0/23    core/src/io/iges_reader.cpp
  0.0%   0/11    core/src/io/brep_reader.cpp
 13.3%   2/15    core/src/mesher_trace.cpp
 20.0%   8/40    core/src/io/format.cpp
 23.0%  35/152   core/src/io/mesh_writers.cpp
 29.4%  32/109   core/src/io/occ_writers.cpp
 30.6%  11/36    core/src/io/params.cpp
 34.1%  15/44    core/src/io/probes.cpp
 59.1%  13/22    core/src/io/occ_rollback.cpp

This adds tests/test_io_export.cpp (new weft_io_tests binary, ctest name io_export), covering the format table, param groups, content/extension probes, the writer/reader registry and its pairing rules, glTF/FBX/STL/OBJ/PLY writers, STEP/IGES/BREP round trips, tessellate-and-convert, viewport edge sampling, and the generation debug log. The same measurement after the new suite:

 99.6% export_gltf   100% export_fbx   100% iges_reader(95.7%)  100% brep_reader
 97.4% mesh_writers  93.6% occ_writers  100% params  100% probes  100% format
 100% occ_rollback   100% mesher_trace  97.9% io/system  96.0% viz

Assertions check binary structure rather than golden blobs — e.g. the .glb container header/chunk lengths are validated, accessor counts are tied to the mesh (positions == normals == faceIds, indices == 3 * triangles), the POSITION block's bounds must equal the mesh bounds, normals must be unit length, and _WEFT_FACE_ID values must fall in the model's face-id range. Body naming is asserted against model.solidNames (imported STEP names) with the sanitize/fallback path (object_<solidId>) covered separately.

Fixtures come from makeFixture → writeStep → loadStep → generate, so no new test assets are committed; files are written under TMPDIR. Nothing in core/ changed.

Remaining lowest-coverage modules after this change are core/src/fixture.cpp (71.5%) and core/src/edit.cpp (74.3%), both untouched here.

Link to Devin session: https://app.devin.ai/sessions/02f8dd75cefa4be5848e082b0bc1d368
Requested by: @jmossymoss

[devin-ai-integration]

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