refactor(tess): move per-entity tessellation into entities/

Each entity's tessellation now lives alongside its definition in
entities/*.rs, attached to the entity type via a local trait:

  LegacyTess     — Viewport / Insert / Hatch / Ole2Frame
                   (fallback path for entities not on the truck pipeline)
  LeaderTess     — Leader
  MultiLeaderTess — MultiLeader
  DimensionTess  — Dimension (carries its dim-only helpers too:
                   ~1700 lines of dim_geometry, dim_text_*, format_*,
                   tolerance, suppression, etc.)

scene/tessellate.rs keeps only the cross-entity dispatcher plus the
shared dim machinery reused by leader/multileader/dimension
(ArrowKind, DimGeom, append_arrow, arrow_from_block, add_segment,
add_polyline, normalized_or, push_tri, color_or_inherit, entity_z,
offset_snap_pts). Cross-entity arc helpers + aci_to_rgba moved to
scene/tess_util.rs.

scene/tessellate.rs shrinks from ~3300 to 809 lines. Compile clean,
all 9 tests pass.

Co-Authored-By: Claude Opus 4.7 <[email protected]>

src/entities/insert.rs

@@ -118,3 +118,27 @@ impl Transformable for Insert {
         apply_transform(self, t);
     }
 }
+
+impl crate::entities::traits::LegacyTess for Insert {
+    fn legacy_geometry(&self, world_offset: [f64; 3]) -> crate::scene::tess_util::LegacyGeometry {
+        let [ox, oy, oz] = world_offset;
+        let ip = Vec3::new(
+            (self.insert_point.x - ox) as f32,
+            (self.insert_point.y - oy) as f32,
+            (self.insert_point.z - oz) as f32,
+        );
+        let s = 0.1_f32;
+        let pts = vec![
+            [ip.x - s, ip.y, ip.z],
+            [ip.x + s, ip.y, ip.z],
+            [ip.x, ip.y - s, ip.z],
+            [ip.x, ip.y + s, ip.z],
+        ];
+        (
+            pts,
+            vec![(ip, crate::scene::wire_model::SnapHint::Insertion)],
+            vec![],
+            vec![],
+        )
+    }
+}