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 <[email protected]>
Claude-Session: https://claude.ai/code/session_01AS5Tn3xieewUSg3uxrMVdL

ATTRIBUTION.md

@@ -30,6 +30,7 @@ Generated-in-code assets are original and have no file to list:
 | crates/ui-egui/src/gpu.rs (paint-callback structure) | CADCraft `crates/ui-egui/src/gpu.rs` | MIT OR Apache-2.0 |
 | crates/mcp/src/server.rs, crates/mcp/src/backend.rs (JSON-RPC framing, MCP lifecycle, control-port bridge) | CADCraft `crates/mcp/src/{server,backend}.rs` | MIT OR Apache-2.0 |
 | crates/mcp/src/tools.rs (tool schema helpers and argument checking) | CADCraft `crates/mcp/src/tools.rs`, GridCraft `crates/mcp/src/tools.rs` | MIT OR Apache-2.0 |
+| crates/io/src/threemf.rs (bounded zip part reading and writing) | GridCraft `crates/xlsx/src/package.rs` | MIT OR Apache-2.0 |
 | LICENSE-MIT, LICENSE-APACHE (licence text) | CADCraft | — |
 
 ## Vendored third-party code