Map SectionSymbol, the last unmapped kind

points is canonical; the host derives the point counts and refreshes the
end/tick/label projections with sync_display_fields when it changes, and
validates that style and view-representation references are existing class
objects. The lifecycle test passes every gate in DWG; DXF reopens the symbol
as an unknown entity because cadcodec dispatches SECTIONLINE only inside
blocks (pinned by a canary and already in the cadcodec write-up).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

plugins/opencad-python/entity_manifest.json

@@ -41,7 +41,8 @@
     "Solid3D",
     "Body",
     "Region",
-    "Surface"
+    "Surface",
+    "SectionSymbol"
   ],
   "manual_kinds": [
     "Dimension"
@@ -926,6 +927,47 @@
           "read_only": true
         }
       }
+    },
+    "SectionSymbol": {
+      "fields": {
+        "points": {
+          "required": true
+        },
+        "view_symbol_version": {
+          "read_only": true
+        },
+        "raw_view_symbol_70": {
+          "read_only": true
+        },
+        "version": {
+          "read_only": true
+        },
+        "raw_point_count_90": {
+          "read_only": true
+        },
+        "raw_flags_90": {
+          "read_only": true,
+          "_comment": "Semantics unknown in cadcodec; preserved as read, 0 for a new symbol."
+        },
+        "raw_point_record_count": {
+          "read_only": true
+        },
+        "end_a": {
+          "read_only": true
+        },
+        "end_b": {
+          "read_only": true
+        },
+        "tick_a": {
+          "read_only": true
+        },
+        "tick_b": {
+          "read_only": true
+        },
+        "label": {
+          "read_only": true
+        }
+      }
     }
   }
 }