Back-merge riina #13 — make the OpenSSL wire differential wait for its own server

[ib823]

A sixth riina commit, found by re-checking the mirror before touching it. This one fixes a flake this session hit twice.

Why it matters here, not just there

openssl_wire_differential failed my pre-push gate once and my full-suite run once during this session. Both times it passed in isolation, so I called it a flake and moved on — correct as far as it went, but I never diagnosed it. riina did:

s_server has to bind its own port, so candidate_port binds an ephemeral port and drops it, leaving a window in which a sibling test can take the same number. The old code then polled TcpStream::connect and treated the first success as "the server is up". It is not:

• two tests draw the same port;
• one s_server binds it, the other fails with Address already in use and exits — with status 0, so the exit code says nothing;
• the loser's connect succeeds against the winner's server;
• that server runs with -naccept 1, has already spent its one accept on its own client, and closes;
• the intruding connection is reset and the test dies on the next write.

The existing retry-on-timeout could never fire, because connect succeeded every time. The failure "looked nothing like the port race it was: no timeout, no did not come up, just write_all dying on a broken pipe about 0.2s in."

That is a better diagnosis than "flaky, passes in isolation", which is what I settled for.

Verification

• The previously-flaky suite run 5× consecutively: 7 passed, 0 failed each time
• 3323 Rust tests, 0 failed (+1)
• clippy clean; audit-docs.sh 0 discrepancies, 0 warnings

Cherry-picked cleanly onto main, authorship preserved.

Mirror status

riina/main is now e71160270. It has moved three times during this session's work — c83f0e0c9 → f20e7ba8a (became #82) → dd4c6ff8a (metrics-only) → e71160270 (this).

The standing rule holds: sync-public.sh only with --reconcile, and re-fetch riina and diff its tree before any mirror push. A remembered survey is not sufficient — checking rather than remembering is what caught both #11 and this one.

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