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/DECISIONS/SAFETY-78-drain-at-two.md

@@ -40,4 +40,4 @@ direction. Opposite splits stay forbidden.
 - At N=2 a maintenance CHARGE in a cheap window has the same stall: the
   charging pack's `hi = -floor` against the loop asking the other to give is
   refused on 141 of 261 cycles. The mirror fix is a site `hi`, which can meet
-  the peak floor's `lo` (0 W), so it is not this card.
+  the peak floor's `lo` (0 W), so it is not this card. Fixed by [[SAFETY-84]].

goodwe_controller/DECISIONS/SAFETY-84-charge-at-two.md

@@ -0,0 +1,46 @@
+# SAFETY-84 - a maintenance charge at two inverters
+
+**Status: DECIDED 2026-09-30.** Follows SAFETY-78's "Not changed" list.
+Tests: `test_safety84.py`; identity: `tools/safety84_identity.py`.
+
+**The defect.** In a cheap window the charging pack files its own
+`hi = -charge` (P_MAINT_SHAPE). At N>=2 the loop's site target sat above it
+(a house to cover, or its 400 W slew step from 0 W), so the other pack had to
+give while this one charged. The allocator refuses opposite directions
+(SAFETY-31), so the site sat at 0 W whatever the house
+(`tools/safety84_identity.py` on 9d5b7d1: 436 of 480 cycles refused). N=1
+was fine.
+
+**The fix.**
+- While a pack's own limits make it charge at N>=2, `main.py` files a site
+  limit `charge`, `hi` = what that pack's own `hi` places
+  (`allocator.hi_w()`, whole W as the allocator rounds). The site target is
+  never above it, so the other pack only adds power in the same direction.
+  Opposite splits stay forbidden. Mirror of SAFETY-78's `drain` floor.
+- Peak guard first (owner order, SAFETY-83). The charge is sized to the
+  quarter's headroom under the aim (`charge_headroom_w()`, cap or banked peak
+  minus `peak_margin_w`), so it charges less instead of stopping: the meter
+  sits on the aim and the guard does not need to step in. Where the house
+  alone is over the aim the guard engages, the shaping is suspended as before
+  (`shaping_suspended()`) and no `charge` is filed, so the guard's `lo` and
+  this `hi` never meet in one cycle (no `lo > hi`, no park at 0 W).
+  Without the headroom sizing the aim still held (the suspension did it), but
+  a battery gave power back on 285 of 540 cycles at a 3000 W house
+  (test_safety84.py part 2).
+- A site stop (a safety SET above P_MAINT_SHAPE) sizes the charge to 0 W:
+  the unit `hi` is a hard limit, so a charge under a 0 W stop needs the other
+  pack to give. It was refused, with an ARBITER line, on most cycles of the
+  stop (test_safety84.py part 4). The stop still parked the site at 0 W.
+- A pack that cannot take the charge (a refusal, a full pack, a stale link)
+  files no ceiling; the other pack follows the loop.
+- Not filed at N=1: byte-identical (`tools/safety84_identity.py`, which hashes
+  every claim, in and out of a cheap window and under a banked peak).
+
+**Not changed.**
+- At N=1 the charge is not sized to the headroom: the guard still suspends it
+  and the battery gives until the quarter is safe. The same sizing would fit
+  there; that is an owner question, not this fix.
+- Outside a cheap window (`hi = 0`, solar only) nothing is filed, at any N.
+- A HOLD (SAFETY-86) and the "Battery is not keeping up" alarm (SAFETY-85)
+  are untouched. In the identity runs that alarm's log line comes as often at
+  N=2 as at N=1 (2 in 480 cycles).