MAINT-28 fix-forward: the peak pause ends only on a settled house estimate, and a pack under its peak reserve gives nothing for the house while it holds

Rig FAIL (TEST-23, f3f9483). F-1: the hold ended at each guard step because
grid + site_batt_w added a new meter reading to a battery reading up to 10 s
old. It now ends only when no battery reading may pre-date a step up of its
command (Controller._batts_settled()). F-2: while the pause holds, a pack in
maintenance CHARGE at or under its peak reserve is limited to 0 W, so it gives
only what the guard claims for the peak. Sim: Lag.batt_period_s (a held 10 s
register). Tests: test_maint28b.py. DECISIONS updated.

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

goodwe_controller/app/main.py

@@ -506,6 +506,7 @@ class Controller(InputsMixin, WritesMixin, TradeMixin, CycleMixin, AlarmsMixin,
         self._peak_needed = False
         self._maint_shape_suspended = False
         self._maint_peak_hold = False          # MAINT-28
+        self._m28_step = {}                    # MAINT-28: unit -> (command, step at)
         # OPT-11: battery modes, off by default (SAFETY-10 section 1). Each
         # battery's mode and on/off switch live in the store, not in options,
         # so a change applies at once (`pack_use`, `pack_mode`).