[alex-mextner]

One command for every conversion the libs support: 3d convert IN... OUT, format from the output extension (.stl .3mf .usdz .png; extensionless names filled from the detected format), structured errors listing accepted formats for unsupported routes. Refs #741

What changed (single commit, passed the pre-commit hook before push)

• lib/commands/convert.py + lib/convert_plan.py + lib/convert_mesh.py: self-registering 3d convert command with the collision contract — input-overwrite auto-suffixes, any other existing target refuses (exit 2) with the ready-to-run free-name command unless --force, re-checked at write time via staging+commit.
• Multicolour scad→3mf: OpenSCAD's per-triangle color() 3MF export is read colour-grouped (dimension_model) and re-emitted through orca_project_3mf as one object per colour with per-object extruder metadata — a true Orca/Bambu project 3MF (3d arrange/export: no multi-part 3MF with per-part extruder; arrange explodes multi-colour objects and drops extruder metadata #89's long-standing ask).
• 3mf→3mf retarget alias: convert file.3mf file-u1.3mf --to u1 takes the retarget options and delegates to 3d retarget (--to/--set/--preset/...).
• Routes: scad/stl/3mf/usdz inputs, stl/3mf/usdz/png outputs; usdz route adds usd-core to the tier; png of a 3mf stages through an STL.
• Docs: docs/commands/convert.md in sync with --help, plus README and lib-map entries.

Tests (in this commit)

• tests/test_commands_convert.py — CLI surface: usage/help/exit codes, unknown option, unknown output extension listing accepted formats, one-positional usage error, missing input.
• tests/test_convert_plan.py — collision contract: existing target refuses with the free-name command, --force overwrites, out==input auto-suffixes, write-time re-check.
• tests/test_convert_mesh.py — per-route dispatch: scad→stl through OpenSCAD, lossless stl→stl copy, project-3mf→stl world coordinates, project 3mf→3mf repack, colour-aware scad→3mf worker, usd-core tier, png of scad/3mf, two-input png usage error.
• tests/e2e/test_convert_workflows.py — end-to-end workflows incl. the unzippable multicolour Orca project 3MF.

Rebased onto current main (printer-vision batch #727/#730/#734/#721); only docs/architecture/lib-map.md touched both sides, no conflicts.