feat: multi-forwarder with SRTT-based failover

address accepts string or array, with optional per-server port override.
New fallback pool tried only when all primaries fail. Sequential failover
with SRTT ranking ensures fastest upstream is tried first.

Closes #34 (items 1, 2, 3)

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

src/dot.rs

@@ -362,7 +362,10 @@ mod tests {
             services: Mutex::new(crate::service_store::ServiceStore::new()),
             lan_peers: Mutex::new(crate::lan::PeerStore::new(90)),
             forwarding_rules: Vec::new(),
-            upstream: Mutex::new(crate::forward::Upstream::Udp(upstream_addr)),
+            upstream_pool: Mutex::new(crate::forward::UpstreamPool::new(
+                vec![crate::forward::Upstream::Udp(upstream_addr)],
+                vec![],
+            )),
             upstream_auto: false,
             upstream_port: 53,
             lan_ip: Mutex::new(std::net::Ipv4Addr::LOCALHOST),