fix: CP13-2 — use advertisedIP (routable), not localServerID (opaque)

Bug: setupReplicaReceiver derived the advertised host from localServerID,
which can be an opaque string (from -id flag, e.g., "my-custom-server-id").
This would publish unusable endpoints like "my-custom-server-id:14260".

Fix:
- volume_server_block.go: add advertisedIP field (always a real IP from
  -ip flag), use it instead of localServerID for replica canonicalization
- volume.go: wire *v.ip → blockService.SetAdvertisedIP() at startup
- blockvol.go: StartReplicaReceiver variadic advertisedHost unchanged

Proof (sync_all_bug_test.go TestBug3, 4 sub-cases):
- fallback: wildcard bind without advertisedHost → outbound-IP
- advertisedHost: explicit IP appears in exported addresses
- StartReplicaReceiver_API: public API forwards host correctly
- opaque_identity_not_routable: proves opaque string produces
  non-routable address, confirming production must use advertisedIP

Identity vs transport separation preserved:
- localServerID: stable identity for V2 control (may be opaque)
- advertisedIP: routable IP for transport endpoints (always real IP)

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

weed/command/volume.go

@@ -347,6 +347,7 @@ func (v VolumeServerOptions) startVolumeServer(volumeFolders, maxVolumeCounts, v
 		// volume server. One identity truth across VS, block service, control
 		// bridge, and runtime ownership.
 		blockService.SetServerID(volumeServerId)
+		blockService.SetAdvertisedIP(*v.ip) // CP13-2: routable IP, not opaque ID
 		volumeServer.SetBlockService(blockService)
 	}