- SingleFlightManager now tracks dedupes internally (dedupes++ when
  a request finds an existing leader and enters waitOrRetry)
- Add getDedupes()/resetDedupes() to SingleFlightManager
- Remove recordSingleflightDedupe()/singleflight_dedupes from
  MemoryCacheMetrics (metrics should be pure counters)
- MemoryCacheManager::snapshot() merges metrics + singleflight dedupes
- MemoryCacheStatsCommand uses MemoryCacheManager::snapshot()
- Remove testRecordSingleflightDedupe test