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/app/allocator.py

@@ -344,6 +344,16 @@ class _Narrowed:
         self.lo, self.hi = lo, hi
 
 
+def pin_w(b):
+    """The watts a unit's own SET places once held inside its hard limits,
+    as `plan()` places a standing pin, or None when the unit has no SET."""
+    pv = getattr(b, "pinned_w", None)
+    if getattr(b, "gate", None) is None or pv is None:
+        return None
+    lo, hi = _int_envelope(b)
+    return float(pv) if lo > hi else float(_clamp_int(round(float(pv)), lo, hi))
+
+
 def loop_bound(bounds, live=None):
     """OPT-11. `(lo, hi)`: what the units can place when the LOOP decides.
 
@@ -364,11 +374,8 @@ def loop_bound(bounds, live=None):
             continue
         if getattr(b, "contradiction", False):
             continue
-        pv = getattr(b, "pinned_w", None)
-        if getattr(b, "gate", None) is not None and pv is not None:
-            lo, hi = _int_envelope(b)
-            p = float(pv) if lo > hi else float(_clamp_int(round(float(pv)),
-                                                            lo, hi))
+        p = pin_w(b)
+        if p is not None:
             lo_sum += min(0.0, p)
             hi_sum += max(0.0, p)
             continue