refactor: strip DECNET tokens from container-visible surface

Rename the container-side logging module decnet_logging → syslog_bridge
(canonical at templates/syslog_bridge.py, synced into each template by
the deployer). Drop the stale per-template copies; setuptools find was
picking them up anyway. Swap useradd/USER/chown "decnet" for "logrelay"
so no obvious token appears in the rendered container image.

Apply the same cloaking pattern to the telnet template that SSH got:
syslog pipe moves to /run/systemd/journal/syslog-relay and the relay
is cat'd via exec -a "systemd-journal-fwd". rsyslog.d conf rename
99-decnet.conf → 50-journal-forward.conf. SSH capture script:
/var/decnet/captured → /var/lib/systemd/coredump (real systemd path),
logger tag decnet-capture → systemd-journal. Compose volume updated
to match the new in-container quarantine path.

SD element ID shifts decnet@55555 → relay@55555; synced across
collector, parser, sniffer, prober, formatter, tests, and docs so the
host-side pipeline still matches what containers emit.

tests/service_testing/test_mongodb.py

@@ -14,16 +14,16 @@ import pytest
 from hypothesis import given, settings
 from hypothesis import strategies as st
 
-from .conftest import _FUZZ_SETTINGS, make_fake_decnet_logging, run_with_timeout
+from .conftest import _FUZZ_SETTINGS, make_fake_syslog_bridge, run_with_timeout
 
 
 # ── Helpers ───────────────────────────────────────────────────────────────────
 
 def _load_mongodb():
     for key in list(sys.modules):
-        if key in ("mongodb_server", "decnet_logging"):
+        if key in ("mongodb_server", "syslog_bridge"):
             del sys.modules[key]
-    sys.modules["decnet_logging"] = make_fake_decnet_logging()
+    sys.modules["syslog_bridge"] = make_fake_syslog_bridge()
     spec = importlib.util.spec_from_file_location("mongodb_server", "templates/mongodb/server.py")
     mod = importlib.util.module_from_spec(spec)
     spec.loader.exec_module(mod)