feat(model): 3D solid modelling tab — primitives + boolean ops

Rename the Home tab to "Draw" and add a Model tab beside it with two
groups:

- Model: BOX / CYLINDER / CONE / SPHERE / WEDGE / TORUS, placed with a
  footprint + height. Each is built as a real ACIS Solid3D (acadrust
  primitive builders) plus a truck B-rep, tessellated into the shaded
  mesh pipeline, with edge wires for picking + wireframe.
- Design: UNION / SUBTRACT / INTERSECT via truck-shapeops.

A session-only Scene.model_solids cache holds each solid's truck B-rep
so the boolean tools can combine and chain them. Supersedes the old
command-line BOX/SPHERE/CYLINDER (insert::solid3d_cmds keeps EXTRUDE/
REVOLVE/SWEEP/LOFT).

Known limitations: booleans need solids created this session; curved
ACIS primitives render in-session but not after reload (the SAT
tessellator handles only planar caps); geometry assumes world_offset 0.

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

src/modules/mod.rs

@@ -134,6 +134,7 @@ pub trait CadModule: Send + Sync {
 pub mod annotate;
 pub mod home;
 pub mod insert;
+pub mod model;
 pub mod layout;
 pub mod manage;
 pub mod view;