feat(kad-dht): terminate query paths once the closest-K set is full

[tabcat]

Description

A query path keeps querying every peer closer than the one that returned it,
walking each disjoint path to exhaustion with no convergence check. This adds the
Kademlia/S-Kademlia termination condition: each path tracks the closest peers that
have responded, and once that set is full it stops at peers that can no longer
enter it - both when a peer comes off the queue, and before queuing a newly
discovered one (the latter also avoids the expensive dialability check for peers
it wouldn't query).

The effect is on a cold or sparse routing table, where a lookup explores widely; a
warm table converges in a step or two and the gate rarely fires. Measured against
the public Amino DHT, fresh node per lookup (table seeded only by bootstrap, ~5-7
peers):

peers contacted per lookup	without	with gate
mean	555	245
median	755	218
max	838	440

At kBucketSize = 20 the gate never triggers on the small topologies in the
existing tests, so they're unchanged; it only prunes over-exploration at scale.

Part of #3536.

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation if necessary (this includes comments as well)
• I have added tests that prove my fix is effective or that my feature works