Add .3ds (Autodesk 3D Studio) file format support

[fsalexandre]

Summary

Adds support for loading Autodesk .3ds (3D Studio) models. trimesh 4.x
has no built-in .3ds loader, so this ships a from-scratch chunk parser
(tds_reader.py) plus a 3DS -> OBJ converter (tds_to_obj.py) — mirroring the
existing .kn5 path (convert to a temp OBJ, then reuse _load_obj_with_real_groups
so real per-object names become UI parts and UVs feed the AO bake). No new
dependencies.

Sample output

A real Proton Bus Citiport model (.3ds) rendered through Ortho:

Implementation notes

• Z-up → Y-up: 3DS is Z-up while the renderer is Y-up. tds_to_obj bakes a
  rotation (x,y,z) -> (x,z,-y) (−90° about X — a proper rotation, det +1, so
  triangle winding is preserved). This is scoped only to the 3DS path; no
  other format and no global AxisConfig is touched.
• Robust against corrupt input: non-finite vertex coordinates and
  out-of-range face indices are validated before writing and raise a clean
  ModelLoadError instead of a NaN-blanked render or a raw IndexError.
• Duplicate object names are disambiguated (probed against all emitted names)
  so parts don't silently merge.
• Registered .3ds in SUPPORTED_EXTENSIONS (also enables .3ds inside zips),
  the upload accept filter, and the docs.

Tests

test_3ds.py — happy path (a hand-encoded 3DS as an independent oracle) plus
corrupt-input / duplicate-name robustness cases.

🤖 Generated with Claude Code