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 <noreply@anthropic.com>

src/entities/viewport.rs

@@ -566,3 +566,22 @@ impl Transformable for Viewport {
         apply_transform(self, t);
     }
 }
+
+impl crate::entities::traits::LegacyTess for Viewport {
+    fn legacy_geometry(&self, world_offset: [f64; 3]) -> crate::scene::tess_util::LegacyGeometry {
+        let [ox, oy, oz] = world_offset;
+        let cx = (self.center.x - ox) as f32;
+        let cy = (self.center.y - oy) as f32;
+        let cz = (self.center.z - oz) as f32;
+        let hw = (self.width / 2.0) as f32;
+        let hh = (self.height / 2.0) as f32;
+        let pts = vec![
+            [cx - hw, cy - hh, cz],
+            [cx + hw, cy - hh, cz],
+            [cx + hw, cy + hh, cz],
+            [cx - hw, cy + hh, cz],
+            [cx - hw, cy - hh, cz],
+        ];
+        (pts, vec![], vec![], vec![])
+    }
+}