Prefer low-rtt announce relays

[marcus-pousette-hp]

Prefer lower-RTT announce relays within a small DHT-distance candidate window.

In public-DHT testing this reduced average connect-open time by 117ms (~6%) and p95 connect-open time by 275ms (~11%), while selected relay RTT p95 dropped from 242ms to 37ms.

Previously the announcer used the first 3 closest replies directly, but with this PR: It keeps DHT locality by only considering the first 8 closest replies, then selects the 3 lowest-RTT candidates from that window.

The old behavior could pick high-latency rendezvous relays even when lower-latency candidates were already available nearby.

Testing

Som unit tests where added also below a Public DHT experiment was performed.

Public DHT experiment

Ran public-DHT holepunch attempts using hosted VMs, with both peers forced to firewalled: true so the consistent holepunch path was exercised.

Branch comparison, 30 attempts per branch:

Branch	Success	Open avg	Open p50	Open p95	Data avg	Data p95	Selected RTT avg	Selected RTT p95
main	30/30	1857ms	1819ms	2436ms	1958ms	2481ms	75ms	242ms
fix/rtt-aware-announcer-relays	30/30	1740ms	1723ms	2161ms	1816ms	2342ms	19ms	37ms

Window comparison, 30 attempts per window:

Window	Success	Open avg	Open p95	Data avg	Data p95	Selected RTT avg	Selected RTT p95	Rank avg	Rank p95
3	30/30	1771ms	2340ms	1844ms	2388ms	72ms	242ms	1	2
5	30/30	1733ms	2075ms	1809ms	2121ms	32ms	97ms	2	4
8	30/30	1738ms	2066ms	1792ms	2111ms	17ms	27ms	3	7
10	30/30	1689ms	2274ms	1743ms	2357ms	14ms	25ms	4	9
12	30/30	1733ms	2292ms	1795ms	2340ms	14ms	23ms	5	10
15	30/30	1734ms	2479ms	1794ms	2527ms	13ms	23ms	7	13
20	30/30	1672ms	2043ms	1717ms	2087ms	9ms	20ms	9	19

Selected RTT is the RTT from the announcing node to the DHT nodes selected as announce/rendezvous relays.

Rank is the selected node position in closestReplies; lower rank means closer to the DHT target.

Window 8 (and what this PR chooses as the default value now), is a conservative cutoff: it captures most of the selected-RTT improvement over the current behavior, while avoiding the larger DHT-rank drift seen with bigger windows.

Co-written with AI