feat(32-02): record events and content descriptors (tracer)

- pact.RecordSaved/RecordDeleted/RecordChange, fired as pointers after commit
- pact.ContentDescriptor and request/result types; MenuItemType.Content
- AdminFormSave.Changes carries the owner's changes to extension Save
- cabana.EmitRecordSaved/EmitRecordDeleted; admin save snapshots columns and
  translated values and fires RecordSaved through lagoon.AfterCommit
- signpost validates Content and gains ContentTypes, ContentTypeForModel, Describe

modules/pact/capabilities.go

@@ -546,7 +546,10 @@ type AdminFormLoad struct {
 // the JSON body (numbers as json.Number), a translatable field's a
 // map[string]string of locale to text, a plugin field type's the value its
 // Decode returned. Save runs on every create and update of a record whose
-// administrator sees the extension, with or without values.
+// administrator sees the extension, with or without values. Changes lists
+// what the owner's save changed on the record, as RecordSaved.Changes will
+// (a translated slug once per changed locale), so Save sees a slug change
+// inside the save transaction.
 type AdminFormSave struct {
 	DB         *gorm.DB
 	Controller string
@@ -554,6 +557,7 @@ type AdminFormSave struct {
 	ID         string
 	Created    bool
 	Values     map[string]any
+	Changes    []RecordChange
 }
 
 // AdminFormSaveResult is Save's answer. Notices are non-blocking messages per
@@ -944,7 +948,9 @@ type DependentOptionsProvider interface {
 // item generates its children from records. References, when set, returns the
 // records an item of the type can point at as a tree; nil means the type takes
 // no reference. Resolve is required and turns one item into its URL, title,
-// alternates and children.
+// alternates and children. Content, when set, describes the records behind a
+// record type (ContentDescriptor) for consumers such as sitemaps; nil means
+// the type takes no part in them.
 type MenuItemType struct {
 	Key            string
 	Label          string
@@ -955,6 +961,7 @@ type MenuItemType struct {
 	DynamicItems   bool
 	References     func(ctx context.Context) ([]MenuItemReference, error)                   `json:"-"`
 	Resolve        func(ctx context.Context, in MenuItemResolve) (*ResolvedMenuItem, error) `json:"-"`
+	Content        *ContentDescriptor                                                       `json:"-"`
 }
 
 // MenuItemReference is one record a menu item can point at: the stored