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/resolver.go

@@ -83,6 +83,8 @@ type Result struct {
 	// TypeKey and RecordID name the record behind the URL, "" when none.
 	TypeKey  string `json:"-"`
 	RecordID string `json:"-"`
+	// Merged names the generated values custom JSON-LD replaced.
+	Merged []schema.Merged `json:"-"`
 }
 
 // SEO is the head metadata of a resolved page.
@@ -157,6 +159,9 @@ type Resolver struct {
 	// non-production lock (D-22) is not applied. The sitemap filter decides
 	// on these, so staging lists the same addresses as production.
 	production bool
+	// schema replaces the stored seo_schema value of the page when set
+	// (the graph preview of unsaved changes and the MCP graph tool).
+	schema *string
 }
 
 // Resolver returns the service's resolver.
@@ -799,6 +804,7 @@ func (r *Resolver) resolveAt(ctx context.Context, db *gorm.DB, settings models.S
 		if err := svc.structuredData(ctx, db, settings, &res, structuredIn{
 			key: key, id: res.RecordID, rec: rec, entry: entry, content: content,
 			locale: locale, path: path, url: self, home: home, siteName: svc.SiteName(ctx, settings, locale), seo: seo,
+			schemaValue: r.schema, sources: src,
 		}); err != nil {
 			return Result{}, err
 		}