DateTime.UtcNow.Ticks * 100L overflows long (Ticks ~6.38e17, x100 =
6.38e19 > long.MaxValue 9.22e18), producing corrupted timestamps that
cause PruneExpiredMessages to immediately expire all messages when
MaxAgeMs is set. Use Unix epoch offset to match Go's time.Now().UnixNano().
Port Go store_test.go contract tests for IStreamStore interface using
MemStore. Fix CompactInternal to correctly walk forward to find new
FirstSeq before backward cleanup (matching Go behavior).
21 new tests ported from store_test.go.