The SRTT ordering + failure penalty path was UDP-only, so a DoT primary
in a forwarding-rule pool was never deprioritized on failure and all
DoT entries tied at INITIAL_SRTT_MS in the sort key. With [[forwarding]]
now accepting arrays of upstreams, DoT pools are a first-class case and
need the same healthiest-first behavior the default pool gets for UDP.
- Add Upstream::tracked_ip() → Some(ip) for Udp/Dot, None for Doh
(DoH has no stable IP — reqwest pools connections by hostname).
- Rewire the three SRTT call sites in forward_with_failover_raw.
- Hoist srtt.read() out of the candidate-scoring loop — one lock per
query instead of N (matters now that pools commonly have N>1).
- Drop unused #[derive(Debug)] on UpstreamPool and ForwardingRule.
- Regression tests: udp_failure_records_in_srtt + dot_failure_records_in_srtt.
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