SAFETY-86: peak guard outranks the maintenance hold

During a HOLD the holding battery gives what keeps the quarter at the peak
aim. N=1: the peak floor is refiled at P_MAINTENANCE, as for a drain
(SAFETY-83). N>=2: the hold's pin is raised to the floor minus what the
other live batteries may place, so they give first. A site stop still
decides alone. The hold ends on its clock even if the level dipped.
Below the aim nothing changes (rows identical).

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

goodwe_controller/DOCS.md

@@ -1669,6 +1669,10 @@ 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).
+While a full battery rests after its charge (the hold), it gives just enough
+to keep the 15-minute peak at that aim if the house needs it; with two
+batteries the other one gives first. The hold still ends on its clock
+(SAFETY-86).
 
 #### Capacity-tariff exposure of the grid-following loop