merge: bring the node-identity change forward from phase-8

Three conflicts, all from both branches editing the same passage:

- Main.cpp: kept phase-9's wording. The metrics registry only exists here, so
  "unwinding destroys little: the metrics registry, whose destructor joins its
  export thread" is the true statement on this branch.
- TESTING.md: kept both paragraphs. They document different things (the
  private [network_id], and the log path plus log_level).
- 05-configuration-reference.md: composed both. The identity is now resolved
  before construction and never empty, so every producer stamps the node key
  from the start; the only divergence left is a wallet that already holds a
  different key, which corrects the tracer alone. Rewrote the earlier
  "three producers" blockquote too: its "no fallback", "first boot ... left
  off" and "Known issue" claims are what this change removes.

One silent break the merge could not flag: makeMetricsRegistryOptions() took
the std::optional<std::string> node key that used to be a constructor
parameter, and that parameter is now the resolved keypair. It takes the base58
string directly, and the constructor derives it from nodeIdentity_, which is
declared before both telemetry_ and metricsRegistry_.

.github/scripts/levelization/results/ordering.txt

@@ -202,6 +202,7 @@ tests.libxrpl > xrpl.nodestore
 tests.libxrpl > xrpl.peerfinder
 tests.libxrpl > xrpl.protocol
 tests.libxrpl > xrpl.protocol_autogen
+tests.libxrpl > xrpl.rdb
 tests.libxrpl > xrpl.resource
 tests.libxrpl > xrpl.server
 tests.libxrpl > xrpl.shamap