fix: canonicalize host in AllocateBlockVolumeResponse (CP13-2 follow-up)

AllocateBlockVolumeResponse used bs.ListenAddr() to derive replica
addresses. When the VS binds to ":port" (no explicit IP), host
resolved to empty string, producing ":dataPort" as the replica
address. This ":port" propagated through master assignments to both
primary and replica sides.

Now canonicalizes empty/wildcard host using PreferredOutboundIP()
before constructing replication addresses. Also exported
PreferredOutboundIP for use by the server package.

This is the source fix — all downstream paths (heartbeat, API
response, assignment) inherit the canonical address.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

weed/server/volume_grpc_block.go

@@ -6,6 +6,7 @@ import (
 	"strings"
 
 	"github.com/seaweedfs/seaweedfs/weed/pb/volume_server_pb"
+	"github.com/seaweedfs/seaweedfs/weed/storage/blockvol"
 )
 
 // AllocateBlockVolume creates a new block volume on this volume server.
@@ -31,6 +32,10 @@ func (vs *VolumeServer) AllocateBlockVolume(_ context.Context, req *volume_serve
 	if idx := strings.LastIndex(host, ":"); idx >= 0 {
 		host = host[:idx]
 	}
+	// Canonicalize: if host is empty (VS bound to ":port"), resolve to routable IP.
+	if host == "" || host == "0.0.0.0" || host == "::" || host == "[::]" {
+		host = blockvol.PreferredOutboundIP()
+	}
 
 	resp := &volume_server_pb.AllocateBlockVolumeResponse{
 		Path:              path,

weed/server/volume_server_block.go

@@ -370,6 +370,9 @@ func (bs *BlockService) setupPrimaryReplication(path, replicaDataAddr, replicaCt
 		return
 	}
 	// Track replication state for heartbeat reporting (R1-4).
+	// These addresses are what the primary ships to — they come from the
+	// master's assignment. They should already be canonical (from
+	// AllocateBlockVolumeResponse), but if not, they'll be reported as-is.
 	bs.replMu.Lock()
 	if bs.replStates == nil {
 		bs.replStates = make(map[string]*volReplState)

weed/storage/blockvol/net_util.go

@@ -27,7 +27,7 @@ func canonicalizeListenerAddr(addr net.Addr, advertisedHost string) string {
 	// Wildcard bind — use advertised host or fallback.
 	host := advertisedHost
 	if host == "" {
-		host = preferredOutboundIP()
+		host = PreferredOutboundIP()
 	}
 	if host == "" {
 		// Last resort: return raw address (will be ":port").
@@ -42,7 +42,8 @@ func canonicalizeListenerAddr(addr net.Addr, advertisedHost string) string {
 //
 // This is a fallback — callers should prefer an explicitly configured
 // advertised host when available.
-func preferredOutboundIP() string {
+// PreferredOutboundIP returns the machine's preferred outbound IP as a string.
+func PreferredOutboundIP() string {
 	conn, err := net.Dial("udp", "8.8.8.8:80")
 	if err != nil {
 		return ""