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_snmp.py

@@ -15,8 +15,8 @@ import pytest
 
 # ── Helpers ───────────────────────────────────────────────────────────────────
 
-def _make_fake_decnet_logging() -> ModuleType:
-    mod = ModuleType("decnet_logging")
+def _make_fake_syslog_bridge() -> ModuleType:
+    mod = ModuleType("syslog_bridge")
     def syslog_line(*args, **kwargs):
         print("LOG:", args, kwargs)
         return ""
@@ -34,10 +34,10 @@ def _load_snmp(archetype: str = "default"):
         "SNMP_ARCHETYPE": archetype,
     }
     for key in list(sys.modules):
-        if key in ("snmp_server", "decnet_logging"):
+        if key in ("snmp_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("snmp_server", "templates/snmp/server.py")
     mod = importlib.util.module_from_spec(spec)