refactor(commands): per-entity MassProps + TextContent traits

Two free helpers in app/commands.rs lived as big match-on-EntityType
blocks: massprop_entity (Circle/Arc/Line/LwPolyline/Ellipse area +
perimeter + centroid, ~140 lines) and entity_text_content /
replace_entity_text (Text/MText/AttributeDefinition/AttributeEntity).

Move each to its own trait in entities/traits.rs and implement
per-entity:

  MassPropsCalc  — Circle, Arc, Line, LwPolyline, Ellipse
  TextContent    — Text, MText, AttributeDefinition, AttributeEntity

EntityTypeOps gets dispatch shims that route to the trait via the
existing macro, so callsites just call entity.mass_props() /
entity.text_content() / entity.replace_text(search, rep).

The remaining free helpers in commands.rs (entity_list_details,
entity_extra_info, entity_type_name, build_data_extraction_csv) are
single-command formatting helpers tightly coupled to LIST / EXTRACT /
text-format output. Distributing them as per-entity methods would
inflate every entity file with command-side formatting concerns
without making the command-side any cleaner — left in place.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/entities/circle.rs

@@ -227,3 +227,16 @@ impl Transformable for Circle {
         apply_transform(self, t);
     }
 }
+
+impl crate::entities::traits::MassPropsCalc for Circle {
+    fn mass_props(&self) -> crate::entities::traits::MassProps {
+        use std::f64::consts::{PI, TAU};
+        let r = self.radius;
+        crate::entities::traits::MassProps {
+            area: PI * r * r,
+            perimeter: TAU * r,
+            cx: self.center.x,
+            cy: self.center.y,
+        }
+    }
+}