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.
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