Add BTF churn diagnostics

[mtg0001]

Summary

• Add lightweight counters for .btf descriptor resolves, outbound rendezvous dials, local rendezvous registration/pairing, and first-message handshake timeouts.
• Tag .btf peer nodes with the address/meeting relay used, then show that relay in node diagnostics.
• Log dial/register/pair transitions under existing network debug logging so churn can be attributed before changing peer behavior.

Why

The churn hypothesis is strong but not proven: some descriptors may point at registrations that are no longer alive, and some paired tunnels may never reach the Bitflash version/message handshake. Fixing that blindly risks training the node around the wrong failure. This PR adds the measurement needed to separate stale descriptors from relay and post-pairing failures.

Testing

• git diff --check
• make -f makefile.mingw tests from MSYS2 UCRT64 in src/

[Bitflash-sh]

This is already in main — it went in as part of #113 (4b6f734), not on its own, because I branched that work off a local test branch that already had yours merged into it. My mistake, and the commit message credits none of it to you. I have noted it on #113.

The counters did exactly what they were for. They ruled out the two hypotheses I could not separate by reading — descriptors resolved 53 times out of 54, and nothing paired and then went silent — which left dialling, failing half the time, and that is what led to the single-slot registration in the relay. The bench that followed put the fix at 65% → 98% paired under contention.

Closing as merged-via-#113.