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import-mesh: documented-neutral scale silently multiplies by 1000 (metres→mm) #855

Description

@ecto

Bug

import-mesh documents scale 1 1 1 as "no scaling", but the loader unconditionally multiplies coordinates by 1000 (URDF metres→millimetres convention), so importing a millimetre STL at documented-neutral scale produces a 1000× part. Workaround in the wild: import-mesh-scaled 0.001.

Found during #844's rana-60c assembly example (see PR #852, which documents the workaround in the example rather than changing behavior mid-PR).

Ask

Pick one and document it loudly:

  • make scale 1 1 1 actually neutral and add an explicit :units m|mm option for the URDF path, or
  • keep the ×1000 default but fix the doc comment and warn when no units are stated.

Either way, add a regression test importing a known-dimension mm STL and asserting its bounding box.

(Related doc-vs-behavior drift, noted in PR #854: cylinder/prism doc comments claim "centered at origin" but both build from z=0 — the docs are wrong, the behavior is relied upon.)

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