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)'.

src/lib/description.ts

@@ -15,14 +15,13 @@ import type { Unit, Group } from './types';
  *           Replace {id:description} tokens with a sentinel \x00DESC:id\x00
  *           (null-byte delimited — can't appear in user-authored text).
  *   Pass 2: replace each sentinel with the Pass 1 output for the referenced id.
- *           Since Pass 1 output contains only sentinels (not live {id:description}
- *           tokens), this is one substitution deep. Any remaining sentinels after
- *           Pass 2 (self-references, unresolvable cycles) are stripped to empty.
+ *           Any sentinels that survive after that substitution are cycles — they
+ *           are resolved as "Label (symbol)" (the {id} expansion) rather than
+ *           leaving an empty slot.
  *
- * Result for a mutual reference (id0 ↔ id2):
- *   - id0's description contains id2's Pass-1 text (with id2's back-reference stripped)
- *   - id2's description contains id0's Pass-1 text (with id0's back-reference stripped)
- *   One level of expansion, then stops cleanly.
+ * Result for a mutual reference (pound ↔ dickload):
+ *   pound's description:    "See dickload (dl)"
+ *   dickload's description: "See pound (lb)"
  */
 
 // Null-byte delimiters can't appear in user-authored strings.
@@ -94,15 +93,21 @@ export function resolveAllDescriptions(
   }
 
   // Pass 2: replace sentinels with Pass 1 output of the referenced id.
-  // Strip any remaining sentinels (self-refs, unresolvable cycles) → empty string.
+  // Any sentinels surviving after that substitution are cycles — resolve them
+  // as Label (symbol) instead of leaving an empty slot.
   const result = new Map<string, string>();
   for (const [id, p1] of pass1) {
     result.set(
       id,
       p1
         .replace(SENTINEL_RE, (_match, refId: string) => pass1.get(refId) ?? '')
-        // Strip any sentinels that survived (self-references or nested cycles)
-        .replace(SENTINEL_RE, '')
+        .replace(SENTINEL_RE, (_match, refId: string) => {
+          const u = unitMap.get(refId);
+          if (u) return `${u.label} (${u.symbol})`;
+          const g = groupMap.get(refId);
+          if (g) return g.label;
+          return '';
+        })
     );
   }
 
@@ -127,5 +132,11 @@ export function resolveDescription(
       const unit = unitMap.get(refId);
       return unit?.description ? resolveFields(unit.description, unitMap, groupMap) : '';
     })
-    .replace(SENTINEL_RE, ''); // strip surviving sentinels
+    .replace(SENTINEL_RE, (_match, refId: string) => {
+      const u = unitMap.get(refId);
+      if (u) return `${u.label} (${u.symbol})`;
+      const g = groupMap.get(refId);
+      if (g) return g.label;
+      return '';
+    });
 }