Finalize rc.13 framework contracts

Formalize update routing, config freeze, and error visibility
Add regression coverage and sync backlog records

error_model.go

@@ -0,0 +1,59 @@
+package laniakea
+
+import "errors"
+
+type classifiedError struct {
+	err          error
+	userVisible  bool
+	internalOnly bool
+}
+
+func (e *classifiedError) Error() string {
+	if e == nil || e.err == nil {
+		return ""
+	}
+	return e.err.Error()
+}
+
+func (e *classifiedError) Unwrap() error {
+	if e == nil {
+		return nil
+	}
+	return e.err
+}
+
+// AsUserError marks err as safe to show to the user through the centralized
+// handler error flow.
+func AsUserError(err error) error {
+	if err == nil {
+		return nil
+	}
+	return &classifiedError{err: err, userVisible: true}
+}
+
+// AsInternalError marks err as internal-only so it will be logged but not sent
+// to the user through the centralized handler error flow.
+func AsInternalError(err error) error {
+	if err == nil {
+		return nil
+	}
+	return &classifiedError{err: err, internalOnly: true}
+}
+
+// IsUserError reports whether err was explicitly marked as user-visible.
+func IsUserError(err error) bool {
+	var classified *classifiedError
+	if !errors.As(err, &classified) {
+		return false
+	}
+	return classified.userVisible
+}
+
+// IsInternalError reports whether err was explicitly marked as internal-only.
+func IsInternalError(err error) bool {
+	var classified *classifiedError
+	if !errors.As(err, &classified) {
+		return false
+	}
+	return classified.internalOnly
+}