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

docs/impl/plans/INITIAL.md

@@ -22,9 +22,10 @@ bus. Below.
 ## Recorder
 
 Reads SocketCAN and gpsd directly. It is tier 1, and the broker is a single point
-of failure that raw capture should not share. When `can-bridge` exists, the
-recorder still owns raw capture; it subscribes to the bus only for decoded
-signals.
+of failure that raw capture should not share (decision 25). When `can-bridge`
+exists, the recorder still owns raw capture; it subscribes to the bus only for
+`yukart/vehicle/*` (`--broker`, `''` to disable), and a broker that is down costs
+those envelopes, never a frame.
 
 ### Session triggers
 
@@ -60,6 +61,9 @@ is the intended second client: `ctl mark crank` for a recovered input loss, and
     can-0001.log.zst           candump -L, closed chunks (5 min or 64 MB), zstd
     can-0002.log               the open chunk, plain
     gnss.ndjson                {"rx": <local receive time>, "msg": <gpsd report>}
+    vehicle.ndjson             {"rx": …, "signal": …, "env": <envelope>} for every message on
+                               yukart/vehicle/* (OBD-II now); "retained": true for the
+                               broker's stored copy, which may predate the session
     events.ndjson              marks, can_link, gnss_link, clock_step, drops, storage_bound
 ```
 
@@ -110,6 +114,35 @@ enough to time individual frames against position.
 - **Copy-off.** Recordings stay on the car until the parked-at-home step in the
   power plan, or a manual `rsync`.
 
+## obd-poller
+
+OBD-II mode 01 (SAE J1979) on `can_pcan`, the single CAN writer. Tier 2.
+
+- **Requests** go to the functional address `0x7DF`, one at a time; the first
+  single-frame reply from `0x7E8`–`0x7EF` answers it (kernel filter: nothing else
+  reaches the process). 100 ms timeout, 10 ms between requests.
+- **Discovery** walks the supported-PID bitmasks (`01 00`, `01 20`, …) whenever the
+  bus wakes; only supported PIDs are polled.
+- **Schedule:** rpm, speed, throttle, load, MAP and MAF every round (~10–13 Hz each
+  with a 4 ms ECU); coolant, intake air and fuel level once a second.
+- **Only while the bus is alive:** the interface's received-frame count moved in
+  the last second. A request into a sleeping bus gets no ACK, and the adapter would
+  resend it until error-passive.
+- **Output:** envelopes on `yukart/vehicle/<signal>`, retained; names, units and the
+  `obd2` source checked against contracts at build. `timestamp` is the reply's
+  kernel receive time, identical to its line in the recorder's CAN log.
+- **State** on `yukart/system/obd-poller`: `ready` with supported PIDs, achieved
+  rates and unanswered count every 5 s; `degraded` with the reason (bus idle, no
+  ECU answers, interface errors); last will `stopping`.
+- `--once`: discover, read each supported PID once, print JSON with reply times.
+- **Not yet:** multi-frame replies (ISO-TP flow control), so no DTC reading (modes
+  03/07/0A) and no MUT-III `21 xx` requests. The kernel has no `can-isotp`; flow
+  control will be done by hand.
+
+Tested 2026-10-12 against a fake ECU on `vcan0`: 7 PIDs discovered and polled,
+540 requests and 540 replies in the recorder's log, every envelope's timestamp
+equal to its reply frame's.
+
 ## orin-status
 
 The Orin's own health as envelopes on `yukart/host/<signal>`, retained, once a