feat(entities): surface remaining 2D + Region fields in properties

Closes out the 2D-entity audit (and the Region partner that ships next
to Solid3D / Body). Round-trip-only metadata fields plus a few editable
ones gain Properties-panel rows so the values the parser populates are
visible and don't get silently dropped on save:

- Hatch: seed_points count, pixel_size, normal
- Polyline2D: smooth_surface, default_start_width, default_end_width,
  thickness, normal (with editors for the widths/thickness)
- Region: uid, silhouettes (shared file with Solid3D / Body)
- Underlay: clip_inverted
- RasterImage: class_version, definition_handle, definition_reactor_handle
- Wipeout: class_version, clip_mode, definition_handle,
  definition_reactor_handle
- Leader: annotation_handle, override_color
- Tolerance: dimension_style_name, dimension_style_handle, dimension_gap,
  text_height (editable), direction, normal
- MLine: style_handle, style_element_count, justification picker
- Ole2Frame: version, source_application, binary_data size, dwg_mode,
  dwg_trailing_byte, is_paper_space
- Viewport: snap_base/spacing/angle, twist_angle, front/back clip Z,
  circle_sides, grid_flags, grid_major, base_ucs_handle, ucs_ortho_type,
  background_handle, shade_plot_handle, visual_style_handle,
  default_lighting, default_lighting_type, ambient_color (with editors
  for the snap / clip / circle_sides values)

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

src/entities/leader.rs

@@ -227,6 +227,25 @@ fn properties(leader: &Leader) -> PropSection {
         // Stats
         ro("Vertices", "vertex_count", n.to_string()),
         ro("Length", "length", format!("{:.4}", leader.length())),
+        // Annotation reference + dim-style colour override (read-only —
+        // they are written by the file and survive a round-trip).
+        ro(
+            "Annotation",
+            "annotation_handle",
+            if leader.annotation_handle.is_null() {
+                "(none)".to_string()
+            } else {
+                format!("{:X}", leader.annotation_handle.value())
+            },
+        ),
+        ro(
+            "Override Color",
+            "override_color",
+            match leader.override_color.rgb() {
+                Some((r, g, b)) => format!("RGB({},{},{})", r, g, b),
+                None => format!("{:?}", leader.override_color),
+            },
+        ),
     ];
 
     // Arrow point (vertex[0])