Packaged as a Home Assistant add-on, with a field guide

Turns the reference RS485 controller into something a technician can install
at a client site: a typed config form instead of YAML, an ingress UI that
names misconfiguration in words, and persistent state that cannot be broken by
a timezone.

Why an add-on rather than YAML packages or blueprints:

- Blueprints cannot create helpers, and the maintenance cycle is a state
  machine whose phase and completion date must survive restarts.
- YAML packages need filesystem access, a configuration.yaml edit and a
  restart - none of which belong in a client install.
- Add-ons authenticate with SUPERVISOR_TOKEN, so there is no long-lived token
  to generate, store or leak on someone else's machine.
- Requires HA OS/Supervised. Container and Core installs cannot run add-ons at
  all, which is a market decision, not an oversight.

The control law and the maintenance machine are pure functions with no Home
Assistant imports, and both ship with runnable checks (22 and 22 assertions).
Every assertion corresponds to a rule whose absence caused an observed failure
on hardware - the saturation duration term, the clamp-before-slew ordering, the
deadband, the sign convention.

One behaviour deliberately differs from the implementation it replaces: when
its inputs go missing this commands 0 W rather than replaying the last
setpoint. The reference version kept replaying, which the hardware watchdog
cannot catch - from the ESP32's side, Home Assistant is still talking to it.

Includes the ESPHome firmware (now parameterised: node name, inverter rating,
watchdog timeout) and the optional RS485 e-stop. FIELD-GUIDE.md carries the
commissioning gates, all judged on the wire rather than on how Home Assistant
looks, plus the written statement a site without an e-stop needs signed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016NckgXecasQb2eSsPYNSW6

README.md

@@ -0,0 +1,73 @@
+# GoodWe RS485 Controller — Home Assistant add-on repository
+
+Replaces a GoodWe battery inverter's vendor controller with a local one: holds
+net grid exchange at zero by emulating the inverter's smart meter over RS485,
+and runs the monthly battery maintenance cycle the vendor box was doing.
+
+```
+repository.yaml            add-on repository metadata
+goodwe_controller/         the add-on
+  config.yaml              manifest + options schema (the per-site config form)
+  app/                     the controller
+    control.py             the control law — pure functions, no I/O
+    maintenance.py         the monthly cycle state machine
+    hass.py                Supervisor/Core API access
+    store.py               persistent state in /data
+    mqtt.py                optional status entities
+    web.py                 ingress UI
+    main.py                orchestration, heartbeat, failsafe behaviour
+  test_control.py          runnable checks — no framework needed
+  test_maintenance.py      runnable checks — walks a full cycle in fake time
+  DOCS.md                  the add-on's documentation tab
+firmware/goodwe-master.yaml  ESPHome config for the T-CAN485
+estop/rs485_log.py           optional RS485 e-stop / bus witness
+FIELD-GUIDE.md             installation, commissioning gates, troubleshooting
+```
+
+## Install
+
+Settings → Add-ons → Add-on store → ⋮ → **Repositories** → add this repository's
+URL, then install **GoodWe RS485 Controller**.
+
+Requires **Home Assistant OS or Supervised**. Add-ons cannot be installed on HA
+Container or Core.
+
+## Read this first
+
+**The inverter holds its last command forever.** It has no meter-timeout of its
+own — a controller that dies mid-command leaves the battery running until a
+human intervenes. Measured on real hardware: 5 kW of discharge held for 113
+seconds after a controller went silent.
+
+Three layers exist because of that, and only the third covers the Home Assistant
+machine itself dying:
+
+1. **ESP32 watchdog** — no fresh setpoint for ~30 s → command 0 W, and keep
+   commanding it.
+2. **Wind-down before firmware updates** — 0 W written before the update starts.
+3. **RS485 e-stop (optional)** — writes 0 W after 30 s of total bus silence.
+
+Sites sold without the e-stop must have the acknowledgement in `FIELD-GUIDE.md`
+§10 signed.
+
+## Development
+
+The control law and the maintenance machine are pure Python with no Home
+Assistant imports, so they run anywhere:
+
+```bash
+cd goodwe_controller
+python3 test_control.py
+python3 test_maintenance.py
+```
+
+Both must pass before shipping any change. They are not unit-test theatre —
+each assertion corresponds to a rule whose absence produced an observed failure
+on real hardware, and the comments in `control.py` say which.
+
+## Compatibility
+
+- GoodWe **ES / BP family** inverters (AA55 / RS485 meter-bus generation)
+- The protocol is **reverse-engineered**. There is no vendor contract, and a
+  firmware change on GoodWe's side could break every installation at once. Say
+  so when you sell it.