SAFETY-84: maintenance charge at N=2 no longer stalls

At N>=2 a cheap-window maintenance charge files a site `hi` (`charge`,
P_MAINT_SHAPE) equal to what the charging pack's own `hi` places, so the
other pack never has to give against it. The charge is sized to the
quarter's headroom under the peak aim so it yields to the peak guard, and
to 0 W under a site safety stop. N=1 byte-identical (safety84_identity).

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

goodwe_controller/DOCS.md

@@ -1666,6 +1666,9 @@ 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.
 If the house would still go over the peak limit, the battery being emptied
 gives more than its drain rate, with one battery too (SAFETY-83).
+In a cheap window the battery being filled charges and the others only add
+charge; none gives while it charges. The charge is cut to what keeps the
+15-minute peak at the limit minus `peak_margin_w` (SAFETY-84).
 
 #### Capacity-tariff exposure of the grid-following loop