fix: cycle references resolve to Label (symbol) instead of empty

When a {id:description} reference forms a cycle, the back-reference
now expands to 'Label (symbol)' rather than being stripped.
So pound ↔ dickload renders as 'See dickload (dl)' and 'See pound (lb)'.

data/units.json

@@ -7,7 +7,7 @@
       "symbol": "lb",
       "group": "dickloads",
       "toBase": 0.038,
-      "description": "{dickload:description}"
+      "description": "My description is {dickload:description}"
     },
     {
       "id": "dickload",