fix: add OBD_SKIP_VOLTAGE_CHECK support for clone ELM327 adapters

Counterfeit ELM327 chips (common in cheap PL2303 USB adapters) often report
bad/zero voltage even when the OBD connection is working. python-obd's
check_voltage=True default rejects these. Add OBD_SKIP_VOLTAGE_CHECK=1 env
var to pass check_voltage=False to obd.OBD(), documented in yaris-obd.service.

Also document Phase 24.6 Yaris 2013 compatibility analysis in PLAN.md.

Co-authored-by: Copilot <[email protected]>

PLAN.md

@@ -1335,6 +1335,44 @@ including two UX polish passes and multiple bug-fix rounds.
 - `app_cellular.py` `r"\r\n"`: SIMCOM SIM868 AT+HTTPPARA="USERDATA" spec requires literal 4-char `\r\n` sequence, not actual CRLF — intentionally correct
 - `app_alerts.py` Authorization header: View tool masked `Bearer {token}` as `******` — actual code is correct
 
+### Phase 24.6: Yaris 2013 OBD compatibility analysis & hardening — ✅ DONE
+**Completed 2026-08-10.** Full compatibility analysis of every OBD PID against the 2013 Toyota Yaris (1.5L 1NZ-FE). Two real bugs fixed, deployed to Pi.
+
+#### PID compatibility verdict
+
+| PID | Command | OBD-II mandatory? | 2013 Yaris | Null-safe in code? |
+|-----|---------|------------------|-----------|-------------------|
+| 0x0D | SPEED | ✅ Yes | ✅ Yes | ✅ Yes |
+| 0x0C | RPM | ✅ Yes | ✅ Yes | ✅ Yes |
+| 0x05 | COOLANT_TEMP | ✅ Yes | ✅ Yes | ✅ Yes |
+| 0x11 | THROTTLE_POS | ✅ Yes | ✅ Yes | ✅ Yes |
+| 0x2F | FUEL_LEVEL | ⚠️ Optional | ✅ Likely | ✅ Yes |
+| 0x01 | STATUS/MIL | ✅ Yes | ✅ Yes | ✅ Yes |
+| Mode 3 | GET_DTC | ✅ Yes | ✅ Yes | ✅ Yes |
+| 0xA6 | ODOMETER | ❌ Non-standard | ⚠️ Likely unsupported | ✅ Fixed (was broken) |
+
+**Odometer note:** PID 0xA6 is a Mode 01 extended PID added in SAE J1979 DA (2014). The 2013 Yaris predates this spec; Toyota's ECU exposure of 0xA6 is inconsistent even on newer models. Expect `odometer = None` permanently — the UI handles this gracefully. The risk was in the decoder raising an exception that killed the whole OBD session; now fixed.
+
+#### Protocol & connection
+- **Protocol:** `OBD_PROTOCOL` env var defaults to `None` → python-obd sends `ATSP0` (auto-detect) → ELM327 will settle on ISO 15765-4 CAN 11-bit 500kbps, which is what the 2013 Yaris uses. ✅
+- **OBD_PROTOCOL=6 hint:** After first confirmed working connection on the real car, uncomment `#Environment=OBD_PROTOCOL=6` in `deploy/yaris-obd.service` to skip the 3–5s auto-probe on every ignition-on.
+- **Port:** auto-detects `/dev/ttyUSB0` (first USB serial device). ELM327 USB adapters enumerate there. Can override with `OBD_PORT` env var. ✅
+- **Reconnect cycle:** ~10s (poll interval) + ~3–5s (ELM327 auto-probe) + ~2s (first poll) = up to ~17s from ignition-on to first live data. Acceptable.
+
+#### Motion detection & trip logic
+- **Primary:** OBD speed (PID 0x0D, polled every 2s) via `get_fresh_speed(max_age=5)`.
+- **Fallback:** GPS haversine displacement when OBD speed is stale/unavailable.
+- **During OBD warmup:** `speed_updated_at is None` → `get_fresh_speed()` returns `None` → GPS fallback activates. No false trip start — idle GPS drift rows spaced 60s apart are filtered by `MIN_GPS_MOTION_SAMPLES=3`.
+- **Trips are post-computed** from raw `tracking` rows — no real-time open/close state machine. Power cut mid-trip is safe; all committed rows are recovered on restart. ✅
+
+#### Bugs fixed
+
+| File | Severity | Bug | Fix |
+|---|---|---|---|
+| `modules/app_obd.py` `_decode_odometer` | HIGH | Raised `ValueError` on unexpected byte count from unsupported/garbage 0xA6 response. Exception propagated through `_query()` and triggered `_mark_connection_lost()`, killing the entire OBD session on 2013 Yaris | Wrapped in `try/except Exception`, return `None` on any error |
+| `modules/app_obd.py` `connect_obd` | MEDIUM | All sensor globals retained stale values from the previous ignition cycle — dashboard briefly shows wrong readings on reconnect | Reset all sensor globals to `None`/`[]` at start of each new connection |
+| `deploy/yaris-obd.service` | INFO | No hint for pinning OBD protocol after initial test | Added commented `#Environment=OBD_PROTOCOL=6` with instructions |
+
 ### Phase 25: Pre-install Yaris 2013 compatibility test — 🚧 PLANNED (core-system gate)
 **Added 2026-08-09; intentionally immediately before the Android app.** Bench tests, synthetic
 routes, attached-hardware tests, and stationary Pi runs cannot prove the complete system works