CLDC-4145: Derive is partner for under 16s

in contrast, clear is partner if the age goes >=16 so the user must answer the question again

app/models/derived_variables/lettings_log_variables.rb

@@ -75,7 +75,7 @@ module DerivedVariables::LettingsLogVariables
       self.beds = nil
     end
 
-    clear_child_ecstat_for_age_changes!
+    clear_child_constraints_for_age_changes!
     child_under_16_constraints!
 
     self.hhtype = household_type
@@ -269,14 +269,16 @@ private
     (2..8).each do |idx|
       if age_under_16?(idx)
         self["ecstat#{idx}"] = 9
+        self["relat#{idx}"] = "X" if form.start_year_2026_or_later?
       end
     end
   end
 
-  def clear_child_ecstat_for_age_changes!
+  def clear_child_constraints_for_age_changes!
     (2..8).each do |idx|
-      if public_send("age#{idx}_changed?") && self["ecstat#{idx}"] == 9
-        self["ecstat#{idx}"] = nil
+      if public_send("age#{idx}_changed?")
+        self["ecstat#{idx}"] = nil if self["ecstat#{idx}"] == 9
+        self["relat#{idx}"] = nil if self["relat#{idx}"] == "X" && form.start_year_2026_or_later?
       end
     end
   end

app/models/form/lettings/questions/person_partner.rb

@@ -27,4 +27,8 @@ class Form::Lettings::Questions::PersonPartner < ::Form::Question
 
     base_question_number + (4 * @person_index)
   end
+
+  def derived?(log)
+    form.start_year_2026_or_later? && log.is_partner_inferred?(@person_index)
+  end
 end

app/models/lettings_log.rb

@@ -403,6 +403,10 @@ class LettingsLog < Log
     unittype_gn_changed? && unittype_gn_was == 2
   end
 
+  def is_partner_inferred?(person_index)
+    public_send("age#{person_index}") && public_send("age#{person_index}") < 16
+  end
+
   def is_shared_housing?
     # 4: Shared flat or maisonette
     # 9: Shared house