3MF: export, and 3MF/STL import as mesh bodies

Export: ExportCommand/CLI --out/MCP export with .3mf write a 3MF Core package
([Content_Types].xml, _rels/.rels, 3D/3dmodel.model; millimetres), one
mesh object and build item per body, named after it, vertices shared and
welded watertight (T-junctions between faces split), base-material colours
when bodies carry one (STEP colours now ride on bodies). Tessellation as
for STL. File > Export 3MF.

Import: doc.open/drag-drop/CLI/MCP open accept .3mf and .stl; Insert Mesh
(ParaMeshInsertAlignCommand) adds them to a design as a MeshImport feature.
Kernel mesh bodies render, measure exactly, move and export; solid
features refuse them with a clear error. 3MF reading flattens components
and build transforms, converts units, reads colours; sizes, entries,
nesting and element counts are capped.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AS5Tn3xieewUSg3uxrMVdL

Cargo.toml

@@ -37,6 +37,10 @@ truck-meshalgo = { version = "0.4", default-features = false, features = ["tesse
 truck-stepio = { version = "0.3", default-features = false }
 truck-topology = "0.6"
 
+# packages (3MF)
+zip = { version = "3", default-features = false, features = ["deflate"] }
+quick-xml = "0.38"
+
 # images
 image = { version = "0.25", default-features = false, features = ["png"] }