Face-tagged 3MF exchange: format spec, reader, geometry.import.mesh

docs/formats/tagged-mesh.md defines the exchange: 3MF Core with an optional
ft:face attribute per triangle (and ft:faces per object) in a ModelCraft
namespace. modelcraft_geometry::tagged_mesh::read_3mf reads it from bytes
(wasm), capping every size and count and range-checking every index and
coordinate. geometry.import.mesh adds the surfaces to the session, which keeps
them beside the open design and restores them when a command fails.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

crates/geometry/src/lib.rs

@@ -21,6 +21,7 @@ mod design;
 pub mod sample;
 
 pub use design::{Design, FeatureError, MAX_DESIGN_BYTES, open_design, open_design_file};
+pub mod tagged_mesh;
 use solidcraft_kernel::{Body, KernelError, guard};
 
 #[derive(Debug, Clone, PartialEq, thiserror::Error)]