Fix cross-network block detection and remove false orphan artifacts

- Change OrphanedAt from time.Time to *time.Time so JSON omitempty
  correctly omits zero values (Go serializes zero time.Time as
  "0001-01-01T00:00:00Z" which JS treats as truthy)
- Rewrite reconcileOnce with 4-pass architecture: enrichment, reorg
  detection via GetBlock verification, false-positive un-orphaning,
  and legacy block stamping
- Use inferOtherChain() to stamp cross-network blocks with the correct
  chain identifier (testnet4 reports as "testnet4", not "test")
- Move tailer startup after aggregator's first refresh so blocks are
  always stamped with the current chain
- Add StampChain store method, debug-blocks admin endpoint, and
  comprehensive reconcile tests for cross-network scenarios

api/cmd/kamado-api/main.go

@@ -112,16 +112,10 @@ func main() {
 			}
 		}
 	}
-	go tailer.Run(ctx)
-	go agg.IngestBlockEvents(ctx, tailer.Events)
-	go agg.IngestAttemptEvents(ctx, tailer.Attempts)
-
-	// Wait briefly for the aggregator's first refresh to complete so
-	// the very first /api/snapshot or /api/health hit doesn't see an
-	// all-zeros snapshot and report bitcoin_ok=false during its own
-	// initialization. Cap the wait so a permanently-down bitcoind
-	// can't block startup forever — /healthz is honest about the
-	// degraded state.
+	// Wait for the aggregator's first refresh before starting the log
+	// tailer. This guarantees currentChain is set before any block
+	// events are processed, so blocks are always stamped with the
+	// correct chain — preventing false orphans on network switches.
 	select {
 	case <-agg.Ready():
 		log.Info("first snapshot ready")
@@ -131,6 +125,10 @@ func main() {
 		return
 	}
 
+	go tailer.Run(ctx)
+	go agg.IngestBlockEvents(ctx, tailer.Events)
+	go agg.IngestAttemptEvents(ctx, tailer.Attempts)
+
 	srv := &http.Server{
 		Addr:              cfg.ListenAddr,
 		Handler:           api.Handler(),