dungeon cages/retraints aren't assignable

docs/changelog.txt

@@ -60,6 +60,7 @@ Template for new versions:
 - `sort`: new search widgets for Info panel tabs, including all "Creatures" subtabs, all "Objects" subtabs, "Tasks", the "Work details" subtab under "Labor", and the "Interrogate" and "Convict" screens under "Justice"
 
 ## Fixes
+- `zone`: don't show animal assignment link for dungeon cages/restraints
 
 ## Misc Improvements
 - `overlay`: allow ``overlay_onupdate_max_freq_seconds`` to be dynamically set to 0 for a burst of high-frequency updates

plugins/lua/zone.lua

@@ -961,9 +961,13 @@ CageChainOverlay.ATTRS{
 
 local function is_valid_building()
     local bld = dfhack.gui.getSelectedBuilding(true)
-    return bld and bld:getBuildStage() == bld:getMaxBuildStage() and
+    if not bld or bld:getBuildStage() ~= bld:getMaxBuildStage() then return false end
-        (bld:getType() == df.building_type.Cage or
+    local bt = bld:getType()
-         bld:getType() == df.building_type.Chain)
+    if bt ~= df.building_type.Cage and bt ~= df.building_type.Chain then return false end
+    for _,zone in ipairs(bld.relations) do
+        if zone.type == df.civzone_type.Dungeon then return false end
+    end
+    return true
 end
 
 local function is_cage_selected()