feat: carry the register input on RegisterEvent and export the registration steps

- RegisterEvent.Payload: a copy of the register input without password and
  password_confirmation (Winter's golem15.user.register data array)
- RegisterUser, RegisterOptions, FireRegisterEvent, IssueToken and APIArray:
  the steps POST /register runs, now called by Register itself; its status
  codes and bodies are unchanged
- README documents the exports and Payload

classes/events.go

@@ -16,7 +16,14 @@ func (e *GetApiArrayEvent) Collected() map[string]any {
 	return e.data
 }
 
-// RegisterEvent is fired after a user row is created. No listener consumes it yet.
+// RegisterEvent is fired after a user row is created (Winter's
+// golem15.user.register).
 type RegisterEvent struct {
 	User *models.User
+	// Payload is a copy of the register input, mirroring the data array
+	// Winter passes with golem15.user.register, so a listener can read
+	// extra keys a client sent (an invitation token, for example).
+	// password and password_confirmation are never included, in plaintext
+	// or hashed. Listeners must treat it as read-only input.
+	Payload map[string]any
 }