Add multi-address fallback for node connectivity

NodeConfig and MasterNodeConfig gain an optional addresses[] field
for fallback addresses tried in order after the primary address.
Provides resilience when Tailscale DNS is down or a node is only
reachable via LAN.

- dialAgentMulti: tries each address with a 3s health check, returns
  first success
- forEachNode: uses multi-address dialing
- AgentPool.AddNodeMulti: master tries all addresses when connecting
- AllAddresses(): deduplicates primary + fallback addresses

Config example:
  [[nodes]]
  name = "rift"
  address = "rift.scylla-hammerhead.ts.net:9444"
  addresses = ["100.95.252.120:9444", "192.168.88.181:9444"]

Existing configs without addresses[] work unchanged.

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

internal/config/master.go

@@ -62,9 +62,24 @@ type TimeoutsConfig struct {
 
 // MasterNodeConfig is a bootstrap node entry in the master config.
 type MasterNodeConfig struct {
-	Name    string `toml:"name"`
-	Address string `toml:"address"`
-	Role    string `toml:"role"` // "worker", "edge", or "master"
+	Name      string   `toml:"name"`
+	Address   string   `toml:"address"`
+	Addresses []string `toml:"addresses,omitempty"`
+	Role      string   `toml:"role"` // "worker", "edge", or "master"
+}
+
+// AllAddresses returns the node's primary address followed by any
+// fallback addresses, deduplicated.
+func (n MasterNodeConfig) AllAddresses() []string {
+	seen := make(map[string]bool)
+	var addrs []string
+	for _, a := range append([]string{n.Address}, n.Addresses...) {
+		if a != "" && !seen[a] {
+			seen[a] = true
+			addrs = append(addrs, a)
+		}
+	}
+	return addrs
 }
 
 // LoadMasterConfig reads and validates a master configuration file.