SAFETY-103: no swing on old battery readings after a drain ends

While a battery settles from its ended drain (SAFETY-95), the house point
was grid + every battery's reading. A register can be 10 s older than the
meter, so B swung 860 -> 1650 -> 870 W on A's old reading (rig check 5).

Now the house point is held and moved only as far as the meter proves:
each battery is between its last changed reading and its commands since
(a reading that changed this cycle, or a refused side, as it reads).
N=1 and 1 s registers are byte-identical to release/1.0.

DECISIONS/SAFETY-103-old-readings-after-a-drain.md, test_safety103.py.

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

goodwe_controller/DECISIONS/SAFETY-103-old-readings-after-a-drain.md

@@ -0,0 +1,79 @@
+# SAFETY-103 - no swing on old battery readings after a drain ends
+
+**Status: implemented 2026-10-08, for review.** Tests: `test_safety103.py`.
+
+**The defect** (SAFETY-95 rig test, check 5, F1). While a battery settles
+from its ended drain, SAFETY-95 steers on the house's point: grid + every
+battery's reading - aim. The meter is 1 s old; a battery register can be
+10 s old (the SAFETY-90 register model). At E+13 A's reading said 1238 W
+(6.8 s old) while A was at 616 W, so the house looked ~620 W bigger: B went
+860 -> 1650 -> 870 W. B's own old reading does the same the other way.
+
+**The rule** (`main.py`, `_track_reads()`, `_drain_grid_w()`, N>=2, only
+while a drain's tail settles):
+
+1. Each cycle, per battery: the range its power can be in. That is its last
+   changed reading, widened by every command on its wire since (a lagging
+   battery moves from one toward the others, never beyond). A new reading
+   starts the range again.
+2. A reading that changed this cycle is taken as it reads. So is the side a
+   battery refuses (SAFETY-73): it is not heading that way.
+3. The house point is held (first cycle: the loop's start from SAFETY-95).
+   It moves only as far as the meter proves: to the nearest value of grid +
+   the batteries' ranges - aim.
+
+So the loop no longer moves on a reading that has not caught up. A real
+house step still moves it, as soon as the meter shows more than the
+batteries can explain.
+
+**Why not the tester's rule** ("a battery whose reading is older than its
+last command counts at its command"). B's command changes almost every
+cycle, so B is almost always at its command. That is the plain meter, which
+chases A's tail: B swings 1420 W. A lag model needs the plant's time
+constant, which is not known for a client's inverter. Measured on the plant
+below (N=2, 1200 W, A Off; swing = a move of the others later undone):
+
+| rule | 10 s lag | 5 s lag | 20 s lag | surplus, 5 s lag |
+|---|---|---|---|---|
+| release/1.0 (readings) | 760 | 840 | 150 | 800 |
+| tester's: at the command | 1420 | 1020 | 1600 | 800 |
+| lag model, tau 10 s | 70 | 140 | 80 | 580 |
+| lag model, tau 5 s, plant 20 s | | | 600 | |
+| hold all window | 0 | 0 | 0 | 0 |
+| **this rule** | **0** | **0** | **0** | **130** |
+
+Holding all window ignores a house step: a 2 kW kettle at E+5 s imported
+14.4 Wh over the next 30 s (release 11.5, this rule 11.3); 2 kW more sun
+exported 16.9 Wh (release 8.5, this rule 8.9).
+
+**Before -> after** (`test_safety103.py`, `tools/safety103_plant.py`: 800
+W/s ramp, 10 s lag, meter 1 s, registers 10 s at the rig's phases, B idles
+at -100 W; summed swing of the others in the 23 s after the end):
+
+| case | release/1.0 | after |
+|---|---|---|
+| rig check 5: N=2, 1200 W, A Off | B 1140 -> 850 -> 1630 -> 870, swing 760 W | B holds 1140 W |
+| N=3, 1200 W / 4000 W, A Off | 680 / 630 W | 0 / 0 W |
+| solar surplus, N=2 / N=3 | 890 / 740 W | 130 / 130 W |
+| lag 5 or 20 s, registers 5 or 10 s, N=2 and N=3 | | at most 10 W |
+| registers as fresh as the meter (1 s) | | rows byte-identical |
+| refusal end | | rows byte-identical |
+| N=1, every end | | rows byte-identical (also against the release/1.0 tree) |
+
+**Cost.** In the window, a house change smaller than what the batteries
+could still be doing is not followed until a reading catches up (at most a
+register period) or the window ends (23 s). Under a surplus the others
+still swing up to 130 W, a third of a slew step.
+
+**Not covered.**
+- Outside the drain tail the loop steers on the meter; readings only reach
+  the saturation freeze (SAFETY-96), unchanged.
+- A lost link or a refusal ends the tail at once (A is out of the split, or
+  reads ~0 W). The swing after them is the loop's own on a lagging plant,
+  the same with 1 s registers and on release/1.0 (up to 1.9 kW at N=3 under
+  a surplus): not this card.
+- A battery that overshoots its command, or a reading offset beyond the
+  range, can put the true power outside the range; the meter then moves the
+  point, as before.
+- The end cycle's start (SAFETY-95 step 2) still uses the readings; the
+  batteries were steady during the drain, so they are current there.