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/arc.rs

@@ -244,3 +244,29 @@ impl Transformable for Arc {
         apply_transform(self, t);
     }
 }
+
+impl crate::entities::traits::MassPropsCalc for acadrust::entities::Arc {
+    fn mass_props(&self) -> crate::entities::traits::MassProps {
+        use std::f64::consts::TAU;
+        let r = self.radius;
+        let span = {
+            let s = (self.end_angle - self.start_angle).rem_euclid(TAU);
+            if s < 1e-6 {
+                TAU
+            } else {
+                s
+            }
+        };
+        // Sector area (pie slice)
+        let area = 0.5 * r * r * span;
+        let arc_len = r * span;
+        // Centroid of arc (chord midpoint direction)
+        let mid_rad = self.start_angle + span / 2.0;
+        crate::entities::traits::MassProps {
+            area,
+            perimeter: arc_len,
+            cx: self.center.x + r * mid_rad.cos(),
+            cy: self.center.y + r * mid_rad.sin(),
+        }
+    }
+}