docs(energy): record the SDM630 power sign, + import / − export

The backend's grid history uses PRAD2 while the live tile uses Deye; a
review asked whether both use the same sign. They do.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

research/energy/README.md

@@ -34,6 +34,10 @@ runs on PV and the grid, so a full battery with ~0 W flow is the normal state.
 
 Fronius Symo 3.7 (`192.168.0.143`, Solar API JSON) produced another 4.4 kWh that day.
 
+House meter SDM630 (`electricity.PRAD2`): `total_power_active` is **+ import / − export**, like the
+Deye grid registers. Checked 2026-10-23 in InfluxDB: −2.5 to −5.5 kW between 11:00 and 15:00 local
+while PV exported, +645 W at night matching Deye's 640 W import.
+
 Considered and rejected (2026-10-23): pointing the logger's second server ("Server B") at our
 backend. Push frames are poorly documented binary data every ~5 min, need a TCP server with
 acks, and a wrong setting could cut off the Solarman cloud my father uses. Local FC3 polling