[naliyi]

Summary

• The private transport now waits for the relay's EOSE on the reply subscription before publishing the request (at most 2 s, then it publishes anyway). The subscription and the request travel on separate connections, so a relay could take the request first and deliver the hub's reply to no one; the client then waited out not_after for an answer that had been sent.
• The test relays now send EOSE after registering a reply subscription, as real relays do.

Seen once in the v0.5.0-rc.24 release run (TestAuditDPrivacy_TimingPairsRequestAndReply timed out); not reproduced locally in 40 runs under load, so the new test reproduces it deterministically instead.

Test plan

• New: a relay that is slow to register the REQ and answers the request at once; the reply still arrives. The no-wait mutant fails it 3/3.
• go vet, go mod tidy, gofmt, go test -race -count=1 ./nipcash/...; nipcash/client back to ~2.7 s