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/DOCS.md

@@ -1704,7 +1704,8 @@ When a battery's link drops, the others take over one slew step a cycle,
 and control does not stall on the dropped battery's power (SAFETY-96).
 When a drain ends (done, Off, timeout, abort, lost link, refused), the
 other batteries cover the house at once but do not export its drain; then
-they move one slew step a cycle (SAFETY-95).
+they move one slew step a cycle (SAFETY-95). While that battery settles,
+an old battery reading does not move them (SAFETY-103).
 
 #### Capacity-tariff exposure of the grid-following loop