fix: prepend event title to Opinion child market titles

Opinion categorical markets produce generic titles like "$200M" or
"June 30, 2026" that lack subject context. This caused the cross-venue
matcher to incorrectly pair them with unrelated markets sharing the
same threshold.

Now applies the same ${event} - ${market} pattern Polymarket uses:
"$200M" → "Probable FDV above ... one day after launch? - $200M"

Author: realfishsam

core/src/exchanges/opinion/normalizer.ts

@@ -353,10 +353,16 @@ export class OpinionNormalizer implements IExchangeNormalizer<OpinionRawMarket,
             price: 0.5,
         };
 
+        const childTitle = child.marketTitle || '';
+        const parentTitle = parent.marketTitle || '';
+        const combinedTitle = parentTitle && childTitle
+            ? `${parentTitle} - ${childTitle}`
+            : parentTitle || childTitle;
+
         const market: UnifiedMarket = {
             marketId,
             eventId: String(parent.marketId),
-            title: child.marketTitle || '',
+            title: combinedTitle,
             description: child.rules || '',
             outcomes: [yesOutcome, noOutcome],
             resolutionDate: new Date(toMillis(child.cutoffAt)),

core/test/unit/normalizers/opinion.test.ts

@@ -328,9 +328,9 @@ describe('OpinionNormalizer.normalizeMarketsFromEvent', () => {
         const markets = normalizer.normalizeMarketsFromEvent(RAW_CATEGORICAL_MARKET);
         expect(markets).toHaveLength(2);
         expect(markets[0].marketId).toBe('201');
-        expect(markets[0].title).toBe('Decrease rates');
+        expect(markets[0].title).toBe('ECB Rates Decision June 2026 - Decrease rates');
         expect(markets[1].marketId).toBe('202');
-        expect(markets[1].title).toBe('No change');
+        expect(markets[1].title).toBe('ECB Rates Decision June 2026 - No change');
     });
 
     test('should set eventId on child markets', () => {