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.

numa.toml

@@ -66,6 +66,13 @@ api_port = 5380
 # [[forwarding]]                                    # DoH upstream: full https:// URL
 # suffix = "example.corp"
 # upstream = "https://dns.quad9.net/dns-query"
+#
+# [[forwarding]]                                    # array of upstreams → SRTT-aware failover
+# suffix = ["google.com", "goog"]                   # fastest-healthy first, dead one skipped
+# upstream = [
+#     "tls://9.9.9.9#dns.quad9.net",
+#     "tls://149.112.112.112#dns.quad9.net",
+# ]
 
 # [blocking]
 # enabled = true               # set to false to disable ad blocking