feat: log OBD speed before GPS fix so rolling starts aren't lost

- _log_loop: when GPS has no fix but OBD speed > 1, write speed-only rows
  (NULL coordinates) instead of dropping the motion
- insert_tracking: accept None gps_x/gps_y -> NULL GPSX/GPSY
- app_trip: keep NULL-coord rows through normalize; _segment returns 0 for
  missing coords; route_points drops them (no position to draw)
- tests: speed-only rows build a trip with max speed + zero distance
- docs: FEAT-12 + readme note; server already filters NULL-GPS safely

readme.md

@@ -50,7 +50,8 @@ Dashboard — mobile (390 px) and desktop (1280 px):
   (create/delete/rotate keys), and server-side device identity sync that hot-reloads on the Pi.
 - **Trip history and replay** — motion-aware detection (OBD wheel speed primary, GPS displacement
   fallback). Metric summaries, offline route polyline, play/pause timeline with moving marker and
-  live reverse-geocoded location names. Tap-to-replay on mobile.
+  live reverse-geocoded location names. Tap-to-replay on mobile. Captures OBD speed before the
+  first GPS fix so rolling starts aren't lost.
 - **Status page** — Pi health (temp, RAM, disk, uptime, throttling), service states, uplink/server/
   GPS/OBD diagnostics, and quick actions (test server, sync now, reconnect uplink, restart app).
 - **Offline-first alerts** — notification bell on every page for check-engine, storage pressure,