SAFETY-91: a held battery is drawn only after the others are full

At N>=2 a battery at or under its own peak reserve now weighs 0 whenever
the site gives (not only for the peak), a draining battery's pin becomes
a floor at its drain rate so it rises before the held one gives, and a
quarter's edge is graded on the next quarter when its first telegram is
late. SAFETY-25 option A stays: the held battery is still drawn once the
others are full. N=1 unchanged.

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

goodwe_controller/DECISIONS/SAFETY-91-held-battery-last.md

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+# SAFETY-91 - a held battery is drawn only after the others are full
+
+**Status: DECIDED 2026-10-01 (owner: held battery last, SAFETY-25 option A
+kept).** Follows SAFETY-87. Tests: `test_safety91.py`.
+
+**The defect** (SAFETY-87 rig test, F1 and F2). At N>=2 a battery at or under
+its own peak reserve was held back (weight 0) only on cycles where the peak
+guard asked for power.
+- F1: B draining at 1000 W, A held at 15 %, house 5500. Once the grid was
+  near zero the loop set the target, A had its normal weight and B's drain
+  pin bound, so the loop drew A at 2500 W for the whole quarter (15.0 ->
+  10.7 %) while B stayed at 1000 W. In the next quarter A reached its
+  cut-out, and B stayed pinned at the peak floor (~2422 W), not its 2500 W.
+- F2: in a HOLD (A holding, B held), at a quarter's edge the guard saw a block
+  with no seconds left and asked for nothing for one cycle. The HOLD's
+  ceiling lapsed, A went back to 0 W and the loop's target went to B: 2500 W
+  for 1 s.
+
+**The fix** (`main.py`, N>=2 only).
+- `_held_back()`: the batteries at or under their own reserve while another
+  can still give (live, not refusing), graded as in `_peak_on_able()`. They
+  weigh 0 in every split where the site gives, not only for the peak. With
+  nobody able (all held, or the rest refusing or dropped) nobody is held back
+  and the split is as before.
+- `_drain_rise()`: while a battery is held back, a draining battery's pin
+  becomes a floor at its drain rate (`lo`), so it rises towards its rating
+  before the held one gives. Not for a held battery's own drain, not under a
+  site stop, not for a battery that is not live.
+- `tariff.next_quarter()`: past a block's end, before the next block's first
+  telegram, the guard grades the next quarter, projected from the held
+  reading (the averager credits it there). Not when that reading is too old
+  to be credited (`max_hold_s`).
+
+**What stays.** SAFETY-25 option A: once every able or draining battery is
+at full power, the loop still draws a held battery (`allocator._fill()`'s
+last-resort pass). With house 5500 and 2 x 2500 W, A still gives 2500 W
+after B is full. Nothing is placed on a battery at its cut-out: OPT-09's
+pack limit closes it (`hi = 0`), and the drain rise lets B take the rest.
+
+**Before -> after** (`test_safety91.py`, 2 x 2500 W, lagging battery 10 s):
+
+| case | before | after |
+|---|---|---|
+| B drain 1000, A held, house 4000 | A 2500, B 1000 | A 1500, B 2500 |
+| same, house 5500 | A 2500, B 1000 | A 2500, B 2500 (option A) |
+| cycles A asked while B not full | 612 of 900 | 0 |
+| A at its cut-out, house 5500: B | 1572 W mean | 2500 W, A 0 W |
+| HOLD, late telegram at the edge | A 0, B 2470 for 1 cycle | A 2050, B 0 |
+| idle, A held, house 2000 | A 1005, B 1005 | A 0, B 2020 |
+| N=1, every scenario | | byte-identical |
+
+**Why N>=2 only.** At N=1 there is no other battery to give first. The edge
+roll would also fix the N=1 one-cycle dip at a quarter's edge (SAFETY-86
+rig, "A dropped to 0 W for 1 s"), but this ticket keeps N=1 byte-identical;
+extending it is a one-line change for a later ticket.
+
+**Re-pinned.** `test_modes.py` `n2` and `n2_off`: from t=1600 battery A is
+at its 20 % reserve, so B carries the house and A gives last (100 of 540
+cycles differ). With SAFETY-87's held rule both are a5dddad's again, and the
+test grades that.
+Two fixtures changed with it: `test_safety87.py` part 10's "without the
+raise" run also turns off `_drain_rise()` (else B rises anyway), and
+`test_peakguard.py` 10g and `test_watchdog.py`'s two-inverter rig give
+their second battery a cut-out, as rule 8 requires (an unknown cut-out is held, so A alone would carry the cycle).
+
+**Not changed.** The peak guard's own rules (SAFETY-87). A drain with
+nobody held stays a pin (SAFETY-78). An Off battery beside a held one
+(SAFETY-87 Open). The first cycle after start, before any quarter is
+measured, can still put the loop's first step on a held battery in a HOLD.
+SAFETY-88, -89, -90, -92, -93, -94.