OPT-11 P3: price runs per battery

Cheap and expensive hours as runs of consecutive firm price slots
(priceruns.py); each battery's role per tick from its mode (modetrade.py):
grid charging for Leftover solar / Cheap hours only, never Solar first;
holding for the expensive runs; selling per battery (pack_sell) above the
spread and the floor. The site trade is sized inside the modes (loop bound)
and the peak guard's headroom; a run edge reseeds the loop. Cheap hours only
and selling are enabled on the Batteries screen and in the HA select
(appended). The site Why row no longer says "held back" when the peak limit
pushes power up (F2). Modes off: byte-identical to 7a00f12 on the trade path.

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

goodwe_controller/DOCS.md

@@ -520,9 +520,26 @@ below.
 Off by default. Turn it on, restart the add-on, and the Batteries screen (and Settings, Simple) gives each battery two controls. They apply at once, with no restart.
 
 - **On or off.** Off asks the battery for 0 W and starts no monthly cycle on it. A cycle that is running stops, and the add-on log says so. The peak limit may still use it, but only for what the other batteries cannot cover.
-- **Mode.** *Solar first* charges from spare solar before the other batteries. *Leftover solar* charges from the solar they cannot take. Both cover the house at any hour. Charging from the grid in cheap hours, and selling, come in a later update.
+- **Mode.**
+  - *Solar first* charges from spare solar before the other batteries and covers the house at any hour. It never charges from the grid and never sells.
+  - *Leftover solar* charges from the solar the others cannot take, and from the grid in the cheapest hours. It saves its power for the expensive hours and covers the house then.
+  - *Cheap hours only* does the same with the grid. Until the solar forecast arrives, *Leftover solar* also tops up from the grid like this.
+- **Sell in expensive hours.** For *Leftover solar* and *Cheap hours only*. In the most expensive hours the battery gives its full power: the house first, the rest to the grid. It stops at *Trading keeps at least* (`arbitrage_soc_floor`).
 
-The peak limit, the safety stops and the monthly cycle always come before a battery's mode.
+With battery modes on, these choices replace the *Charging and selling on price* switch (`arbitrage_enabled`); that switch is only read with modes off.
+
+**Cheap and expensive hours.** They come from the published day-ahead prices, as runs of consecutive 15-minute periods, not a single period.
+- The cheap run is the cheapest stretch long enough to fill the batteries that may charge, at their power.
+- An expensive hour costs at least *Battery cycle cost* (`cycle_cost_threshold_eur`, widened as for trading) more than the cheap run. The batteries hold their power until then.
+- The sale uses the most expensive stretch inside those hours, and never a period below 0.
+- A carried (not yet published) price is never used. A run that has started runs to its end.
+- With no prices, or too small a gap, every battery follows the house.
+
+**What always comes first.**
+- The peak limit, the safety stops and the monthly cycle come before a battery's mode.
+- A grid charge is sized so the 15-minute period stays under the peak limit less its margin (`peak_margin_w`). It starts only at 500 W or more. Once the peak limit stops it, it stays off for the rest of that 15-minute period.
+- While one battery charges from the grid, the others rest, and the house uses the grid.
+- A battery that refuses to charge or give power takes no part in a run in that direction.
 
 **In Home Assistant.** With MQTT set up, each battery also gets a *Mode* list and a *Use* switch, for example `select.goodwe_inverter_a_mode` and `switch.goodwe_inverter_a_use`. They do the same as the Batteries screen. Turn battery modes off and restart, and they are removed.
 
@@ -530,6 +547,8 @@ The peak limit, the safety stops and the monthly cycle always come before a batt
 
 *For installers:* a mode is a limit at priority 35, kept apart from the hard limits. It gives way only as far as a limit above it (the peak guard) needs. A mode range always holds 0 W, so a mode can forbid a direction and never force one. With one inverter the mode also bounds the loop. The record is `DECISIONS/OPT-11-battery-modes.md`.
 
+*For installers:* when a charge or sale ends, the loop restarts at the house's own level rather than at the trade's watts. On the replay model that removes a ~3 kW import spike after a 2.5 kW charge and halves the settling time; about 0.1 × the battery's power still overshoots as export. Selling is the `pack_sell` action (and the *Mode* list gains *Cheap hours only* in Home Assistant, at the end).
+
 ### Reading the grid-following loop: `Loop integrator`, `Loop integrator lead`, `Loop reason`
 
 The control law is a bounded integrator: an accumulator that is **not** the