SMA-10: two idle states, hold 0 W or hand the inverter back

A driver now says whether its release() hands the inverter back
(Driver.hands_back; GoodWe is False, so release() stays write(0) and the
Controller never calls it). command_site_release() sits next to
command_site_zero(): the Stop button and shutdown() (so SIGTERM, restart,
control task death and a late config refusal) hand a hand-back unit back,
once, and zero every other unit as before. Safety stops and refusals still
hold 0 W for every unit. Shadow mode releases a hand-back unit. After a
release nothing is written to the unit (heartbeat and cycle included) until
control starts again. release() is bounded by RELEASE_BUDGET_S (= the write
budget), runs concurrently per unit, and a raising or hanging release on one
unit does not stop the others.

The "set to 0 W" sentences for Stop, restart, shadow mode and the stopped
banner are built from the roster (app/handback.py, EN and NL, words from the
driver); a GoodWe-only roster keeps every old literal. A handed-back unit
shows "Back under <its control>" on Home.

Existing test edited: test_messages.py (two classification entries for the
CRITICAL line that moved into a roster builder, and the new release bound).

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

goodwe_controller/DECISIONS/SMA-10-two-idle-states.md

@@ -0,0 +1,51 @@
+# SMA-10 - two idle states: hold 0 W, or hand the inverter back
+
+**Status: for review.** Tests: `test_sma10_two_idle_states.py`. Source: SMA-9
+report sections 1.4, 3.2; owner decision 2026-10-20 (Q1, "as proposed").
+
+A unit is idle in one of two ways, and the driver says which:
+`Driver.hands_back`. **False (GoodWe):** the add-on holds 0 W, `release()` is
+`write(0)` and the Controller never calls it, so Stop keeps today's 0 W write and
+today's 0 W heartbeat. **True:** `release()` gives the unit back to whatever
+steers it by itself (SMA energy management), once, and then nothing is written
+to it until control starts again.
+
+**Why two.** When the add-on goes away (Stop, shutdown, SIGTERM, restart, task
+death, a config refusal) nobody is left to hold a number, so the unit goes back
+to its own controller. When the add-on itself decides to stop (sign trip, site
+sign, circulating power, a refused or non-finite write, missing readings,
+another writer, an inverter fault) it has just said it cannot trust that
+battery, and handing it to something else would let that something do what it
+likes with it. Those hold 0 W through `write(0)` for every unit, whatever its
+driver. Shadow mode on a unit that hands back uses `release()`: it must leave
+the unit alone, and 0 W would be a command.
+
+**Where.** `Controller.command_site_release()` sits beside `command_site_zero()`
+(unchanged). Stop and `shutdown()` call it; the two maintenance-abort paths keep
+zero (the add-on runs on, the sentence says 0 W for a moment). With no
+hand-back unit it is `command_site_zero()`. `_write_unit()` also refuses to write
+a hand-back unit while the add-on is idle (stopping, stopped, or shadow mode): it
+releases once and returns, so the heartbeat and the cycle's own "stopped" 0 W
+cannot reach it. A safety stop runs while control runs, so it falls through to
+the write. Start re-arms: `released` drops and `last_write` clears.
+
+**Bounds.** `RELEASE_BUDGET_S` equals `WRITE_BUDGET_S` (4 s): units are released
+together, so one slow unit costs the kill window no more than a hung GoodWe
+write does. A release at the bound is abandoned, not awaited. A release that
+raises, hangs or returns false leaves that unit un-handed (the next idle write
+tries again) and does not touch the others. Exit 78 before the Controller
+exists has no driver to release and keeps today's path.
+
+**Words.** Every sentence that said "set to 0 W" for the add-on going away or
+for shadow mode is built from the roster in `app/handback.py` (EN and NL): a
+roster with no hand-back unit gets None and keeps its old literal, so a GoodWe
+site sees no changed byte. A hand-back unit's words (`Driver.words`: `back`,
+`back_nl`) come from its driver. MQTT `status` is unchanged; the Home battery
+row of a handed-back unit says "Back under SMA control" and carries
+`handed_back`.
+
+**Not done here (SMA-4, SMA-5, SMA-11):** any SMA/FoxESS driver, the read-back
+after `release()`, per-driver timing, DOCS.md's safety model and the
+"control stalled" sentence, the site-level "Held at 0 W" tiles in shadow mode on
+a mixed roster, and the MQTT per-unit setpoint of a handed-back unit (it still
+publishes 0).