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/model/mod.rs

@@ -0,0 +1,63 @@
+// Model module — 3D solid modelling.
+//
+//   Model group  : create primitive solids (box, cylinder, cone, sphere, …)
+//                  as ACIS Solid3D entities via acadrust's primitive builders.
+//   Design group : combine solids with truck boolean operations
+//                  (union / subtract / intersect).
+
+pub mod boolean_cmd;
+pub mod primitive_cmd;
+
+use crate::modules::{CadModule, IconKind, ModuleEvent, RibbonGroup, RibbonItem, ToolDef};
+
+pub struct ModelModule;
+
+/// Helper to declare a ribbon tool that fires a named command.
+fn tool(id: &'static str, label: &'static str, glyph: &'static str) -> ToolDef {
+    ToolDef {
+        id,
+        label,
+        icon: IconKind::Glyph(glyph),
+        event: ModuleEvent::Command(id.to_string()),
+    }
+}
+
+impl CadModule for ModelModule {
+    fn id(&self) -> &'static str {
+        "model"
+    }
+    fn title(&self) -> &'static str {
+        "Model"
+    }
+
+    fn ribbon_groups(&self) -> Vec<RibbonGroup> {
+        vec![
+            RibbonGroup {
+                title: "Model",
+                tools: vec![
+                    RibbonItem::LargeTool(tool("BOX", "Box", "▦")),
+                    RibbonItem::LargeTool(tool("CYLINDER", "Cylinder", "⬭")),
+                    RibbonItem::LargeTool(tool("CONE", "Cone", "△")),
+                    RibbonItem::LargeTool(tool("SPHERE", "Sphere", "◯")),
+                    RibbonItem::Dropdown {
+                        id: "MODEL_MORE",
+                        icon: IconKind::Glyph("◰"),
+                        items: vec![
+                            ("WEDGE", "Wedge", IconKind::Glyph("◣")),
+                            ("TORUS", "Torus", IconKind::Glyph("◎")),
+                        ],
+                        default: "WEDGE",
+                    },
+                ],
+            },
+            RibbonGroup {
+                title: "Design",
+                tools: vec![
+                    RibbonItem::LargeTool(tool("UNION", "Union", "⊕")),
+                    RibbonItem::LargeTool(tool("SUBTRACT", "Subtract", "⊖")),
+                    RibbonItem::LargeTool(tool("INTERSECT", "Intersect", "⊗")),
+                ],
+            },
+        ]
+    }
+}