SAFETY-135: a young 15-minute period does not cost the night charge the whole period

With battery modes on, a grid charge the peak guard (or a measured headroom)
stops in the first 60 s of a period no longer latches off for the period: it
may start again once the period is 60 s old, if the guard is clear and the
headroom is over START_W. A stop after 60 s latches as before. In a young
period no charge starts while the batteries' give is still falling (over
START_W within 15 s), since the meter may not show that drop yet. The guard
itself is unchanged, so a real over-aim load is cut on the same cycle.

DECISIONS/SAFETY-135-young-period.md, test_safety135.py.

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

goodwe_controller/app/modetrade.py

@@ -36,6 +36,9 @@ Rules that keep this safe (tests: test_modes_p3.py, test_price_run_fixes.py):
     makes the other batteries stop giving power, so a few watts of charge
     flipping on and off would swing the house between them and the grid
     (measured on the 2026-09-09 replay: 13 flips in 4 minutes at 3.5 kW);
+    SAFETY-135: a stop in a period's first YOUNG_S s holds only until then,
+    not to the period's end, and no charge starts then while the batteries'
+    give is still falling (`young`);
   * selling needs selling on for that battery, the sell run, a price >= 0
     now, and the battery above `arbitrage_soc_floor`; it stops there. The
     peak guard never stops a sell (section 9.1: discharge is never capped);
@@ -69,6 +72,12 @@ PRICE_MODES = (LEFTOVER_SOLAR, CHEAP_HOURS)
 # would stop covering the house for less than they give.
 START_W = 500.0
 
+# SAFETY-135: seconds a 15-minute period is young, and how far back a
+# battery's give counts as maybe not yet on the meter.
+# DECISIONS/SAFETY-135-young-period.md
+YOUNG_S = 60.0
+GAVE_S = 15.0
+
 
 @dataclass(frozen=True)
 class Pack:
@@ -164,7 +173,7 @@ def energy(packs, floor_pct, ceiling_pct, done_charge=(), done_sell=(),
 def decide(runs, now, packs, *, charge_ok, sell_ok, trade_why, price_now,
            headroom_w, floor_pct, ceiling_pct, house_w, charging_now=False,
            blocked=False, done_charge=(), done_sell=(),
-           targets=None, draining=False) -> Tick:
+           targets=None, draining=False, young=False) -> Tick:
     """This tick's ranges, roles and site ask.
 
     `runs` is the kept `priceruns.RunPlan` or None (no fresh prices).
@@ -177,7 +186,8 @@ def decide(runs, now, packs, *, charge_ok, sell_ok, trade_why, price_now,
     it going); `blocked` that the guard or the headroom stopped a charge in
     this 15-minute period. `targets` is the day plan (`charge_to()`).
     `draining` says a battery drains for its monthly cycle, so none may
-    charge from the grid (why "cycle").
+    charge from the grid (why "cycle"). `young` says no charge may start
+    yet in this young period (SAFETY-135, why "young").
     """
     plan_on = runs is not None and runs.on
     in_cheap = plan_on and runs.cheap.holds(now) and runs.charge_useful()
@@ -198,6 +208,8 @@ def decide(runs, now, packs, *, charge_ok, sell_ok, trade_why, price_now,
             why = trade_why
         elif headroom_w is None or not headroom_w > need:
             why = "headroom"
+        elif young and not charging_now:
+            why = "young"
         else:
             charging = True
     se = sellers(packs, floor_pct, done_sell) if in_sell and not charging \