obd-poller: OBD-II mode 01, the single CAN writer; recorder keeps its values

obd-poller asks 0x7DF for the nine obd2 signals, only while the bus is alive,
and publishes envelopes stamped with the reply's kernel time. The recorder
subscribes to yukart/vehicle/* and writes vehicle.ndjson beside the frames.
Tested against a fake ECU on vcan0.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DDwfd6Ki8UfG4hK6W5YpJz

deploy/yukart-obd-poller.service

@@ -0,0 +1,16 @@
+[Unit]
+Description=Yukart obd-poller: OBD-II mode 01 polling, the single CAN writer
+Documentation=gitea:yukart/svc-core
+# Tier 2. Ordered after the broker and nothing else: the adapter comes and goes, and
+# the poller only sends while the bus is visibly alive.
+After=mosquitto.service
+StartLimitIntervalSec=0
+
+[Service]
+User=mikkeli
+ExecStart=/usr/local/bin/yukart-obd-poller
+Restart=always
+RestartSec=2
+
+[Install]
+WantedBy=multi-user.target