SAFETY-78: maintenance drain at N=2 no longer stalls

At two inverters the drain pins its pack and the site target is split
around the pin. Below the pin the other pack had to charge, the allocator
refused the opposite directions, and the site sat at 0 W with an ARBITER
line every cycle (house below or above the drain rate, or a grid charge).

main.py now files a site limit `drain` (lo = what the pin places, at
P_MAINTENANCE) at N>=2 while a pack drains, so the other pack only adds
power in the same direction. A drain that cannot give power waits with
one log line. With battery modes on, no grid charge while a pack drains,
and the other battery's card says why. N=1 is byte-identical.

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

goodwe_controller/DOCS.md

@@ -539,6 +539,7 @@ With battery modes on, these choices replace the *Charging and selling on price*
 - The peak limit, the safety stops and the monthly cycle come before a battery's mode.
 - A grid charge is sized so the 15-minute period stays under the peak limit less its margin (`peak_margin_w`). It starts only at 500 W or more. Once the peak limit stops it, it stays off for the rest of that 15-minute period.
 - While one battery charges from the grid, the others rest, and the house uses the grid.
+- While one battery empties for its monthly cycle, no battery charges from the grid.
 - A battery that refuses to charge or give power takes no part in a run in that direction.
 
 **In Home Assistant.** With MQTT set up, each battery also gets a *Mode* list and a *Use* switch, for example `select.goodwe_inverter_a_mode` and `switch.goodwe_inverter_a_use`. They do the same as the Batteries screen. Turn battery modes off and restart, and they are removed.
@@ -1647,6 +1648,10 @@ under `peak_cap_w`, and if the forecast goes over the cap the charge-only clamp
 is dropped so the battery can shave the peak instead. Money outranks the
 maintenance schedule.
 
+With two or more batteries, the one being emptied gives its drain rate and the
+others only add power when the house needs more; none charges while it drains
+(SAFETY-78). Below the drain rate the rest is exported, as with one battery.
+
 #### Capacity-tariff exposure of the grid-following loop
 
 ⚠️ **This section described the exposure BEFORE SAFETY-07. The loop is now