feat: graph validation and custom JSON-LD

- seo_schema (seoschema, E8) with the C17 server rules: invalid JSON with
  line and column, non-objects, 16 KB, depth 8, a literal <script and a
  type outside the kind's list refused; the value re-serialised
- a custom node with a generated @id merges into it (schema.merged save
  notice), other nodes are appended to the graph
- seo_primary_parent (seoparent) for article types; both values stored
  in the entry data on every locale row
- the seo_graph widget action (E9 data), P1 primary_gone and the P2
  schema type and breadcrumb trail rows
- GET /_seo/api/v1/graph and MCP seo_get_graph; seo_set_url_seo and
  apply_batch accept primary_parent and schema

The graph tool lands with the validation: the operations parity test
binds the seo_graph widget action to its tool.

classes/breadcrumbs.go

@@ -160,3 +160,39 @@ func (s *Service) Trail(ctx context.Context, db *gorm.DB, settings models.Settin
 	}
 	return append(out, schema.Crumb{Name: name, URL: site + path, Path: path})
 }
+
+// ParentOption is one choice of the primary category field (P33-23).
+type ParentOption struct {
+	Value string `json:"value"`
+	Label string `json:"label"`
+}
+
+// ParentOptions lists a record's parents as primary category choices, in
+// descriptor order, labelled with their title in locale (the default
+// locale when "").
+func (s *Service) ParentOptions(ctx context.Context, key, id, locale string) []ParentOption {
+	out := []ParentOption{}
+	reg := s.Registry()
+	if reg == nil || key == "" || id == "" {
+		return out
+	}
+	rec, err := reg.Describe(ctx, key, id, s.UnprefixedLocale())
+	if err != nil || rec == nil {
+		return out
+	}
+	if locale == "" {
+		if db, err := s.DB(ctx); err == nil {
+			locale = s.DefaultLocale(ctx, db)
+		}
+	}
+	for _, ref := range rec.Parents {
+		label := ParentValue(ref)
+		if parent, err := reg.Describe(ctx, ref.Type, ref.ID, s.UnprefixedLocale()); err == nil && parent != nil {
+			if l := contentLocale(parent, locale); strings.TrimSpace(l.Title) != "" {
+				label = l.Title
+			}
+		}
+		out = append(out, ParentOption{Value: ParentValue(ref), Label: label})
+	}
+	return out
+}