feat: accept array of upstreams in [[forwarding]]

Mirrors `[upstream] address` — `upstream` accepts string or array
of strings, builds an `UpstreamPool` and routes queries through
`forward_with_failover_raw` so SRTT ordering and failover apply to
matched `[[forwarding]]` rules the same way they do for the default
pool.

Single-string rules keep their current behavior (one-element pool,
equivalent single-upstream path). Empty array errors at config load.

Addresses item 1 of issue #102. Plan: docs/102_item1.md.

src/forward.rs

@@ -118,7 +118,7 @@ fn build_dot_connector() -> Result<tokio_rustls::TlsConnector> {
     )))
 }
 
-#[derive(Clone)]
+#[derive(Clone, Debug)]
 pub struct UpstreamPool {
     primary: Vec<Upstream>,
     fallback: Vec<Upstream>,