feat(10-02): relation field options and relation saves with labels

- FieldRelationContract/FieldRelationProvider bind every type: relation
  field to a belongsTo foreign key or a belongsToMany pivot; activation
  fails naming plugin, controller and field on a missing or broken contract
- GET /{vendor}/{plugin}/{controller}/fields/{field}/options serves
  {value, label} pages scoped by pact.RelationExtendOptionsQuery, behind
  the controller permission; read-only and non-relation fields are 404
- Saves apply present relation keys after the Before hook: ids are
  revalidated through the same scoped query (422 and full rollback
  otherwise), belongsTo sets the foreign key, belongsToMany replaces pivot
  rows in submitted order with the order column set to the index
- Show, create and update return relation values in data and meta.labels
- A belongsTo on a protected fill key is read-only (D-26)
- One six-segment GET pattern dispatches relation lists and field options,
  which ServeMux cannot register side by side
- Admin OpenAPI documents the options route and RecordEnvelope

pact/capabilities.go

@@ -242,6 +242,13 @@ type RelationExtendManageQuery interface {
 	RelationExtendManageQuery(ctx context.Context, relation string, db *gorm.DB) *gorm.DB
 }
 
+// RelationExtendOptionsQuery optionally narrows the rows a form relation
+// field offers (D-17). The same scoped query revalidates submitted ids on
+// save, so a row it does not return cannot be attached (D-18).
+type RelationExtendOptionsQuery interface {
+	RelationExtendOptionsQuery(ctx context.Context, field string, db *gorm.DB) *gorm.DB
+}
+
 // RelationBeforeLink optionally stamps pivot columns before a link insert.
 type RelationBeforeLink interface {
 	RelationBeforeLink(ctx context.Context, relation string, parent, related any, pivot map[string]any) error