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 <[email protected]>

src/command/mod.rs

@@ -51,6 +51,13 @@ pub enum CmdResult {
     CommitEntity(EntityType),
     /// Commit an acadrust entity to the document and end the command.
     CommitAndExit(EntityType),
+    /// Commit a Model-tab 3D solid: the acadrust entity (for selection /
+    /// persistence) plus its truck B-rep (cached for boolean ops + shaded
+    /// rendering). Ends the command.
+    CommitSolid {
+        entity: EntityType,
+        solid: Box<truck_modeling::Solid>,
+    },
     /// Commit an acadrust entity, end the command, and open the in-place text
     /// editor on it (used by MLEADER to type the annotation after placement).
     CommitAndEditText(EntityType),