feat(export): exact ACIS geometry for planar solids on save

OCS models 3-D solids with truck, but extrude/boolean results carried no ACIS
body, so other CAD apps dropped them on open. Add an exact planar exporter and
run it at save time.

- scene::convert::acis_export::planar_solid_to_sat walks a truck Solid whose
  faces are all planes bounded by straight edges into a vertex/face-ring mesh
  and builds an exact ACIS body via acadrust's build_planar_body. Curved
  faces/edges return None (left for the NURBS path). A signed-volume check
  flips the winding if the shell comes out inside-out.
- OpenCADStudio::sync_truck_solids_to_acis fills in ACIS geometry for every
  cached truck solid that still has none, just before a Save / Save-As, so the
  written DWG/DXF carries real modeler geometry.
- Bump acadrust to 9142a50 (build_planar_body + classic-ACIS AcDs roundtrip fix).

Booleans (already cached as truck solids) now export exact geometry. Extruded
polygons will too once the EXTRUDE command caches its truck solid (a separate
change kept out of this commit because command_driver.rs holds unrelated
in-progress edits).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/scene/convert/mod.rs

@@ -1,5 +1,7 @@
 pub mod acad_to_truck;
 #[cfg(feature = "solid3d")]
+pub mod acis_export;
+#[cfg(feature = "solid3d")]
 pub mod acis_to_truck;
 /// Without `solid3d` (e.g. wasm) ACIS/SAT meshing is unavailable; the entry
 /// point stays so callers compile, returning no mesh.