Keep models of rooms removed in Options

Saving wrote only the currently configured rooms, so removing a room in
Configure (even by accident) discarded its learned model at the next
save, and adding it back started from scratch.

Keep validated models of removed rooms (at most 24, oldest dropped
first) in an optional retired_learners payload key and restore them when
the room is added back. Payload schema_version stays 2: the key is
additive and older 0.2.x readers ignore it. Replacing the relay still
starts a fresh model, as before. Status lists retired_room_models.

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

tests/ha/test_lifecycle.py

@@ -182,3 +182,26 @@ async def test_relay_listener_integrates_changes_between_polls(hass, entry, inpu
     hass.states.async_set("switch.boiler", "on")
     await hass.async_block_till_done()
     assert not coordinator._relay_events
+
+
+async def test_removed_room_model_survives_options_round_trip(hass, entry, inputs, hass_storage, freezer):
+    from custom_components.heating_mpc.physics import Learner
+    key = f"{DOMAIN}.{entry.entry_id}"
+    hass_storage[key] = {"version": 1, "data": {
+        "schema_version": 2, "heater_entity": "switch.boiler",
+        "learners": {"sensor.room": Learner(count=250, on_count=125, off_count=125).as_dict()}}}
+    hass.states.async_set("sensor.new", "21", {"unit_of_measurement": "°C"})
+    await setup(hass, entry)
+    hass.config_entries.async_update_entry(entry, options={"room_entities": ["sensor.new"]})
+    await hass.async_block_till_done()
+    future = dt_util.utcnow() + timedelta(minutes=5, seconds=1)
+    freezer.move_to(future)
+    async_fire_time_changed(hass, future)
+    await hass.async_block_till_done()
+    saved = hass_storage[key]["data"]
+    assert set(saved["learners"]) == {"sensor.new"}
+    assert saved["retired_learners"]["sensor.room"]["count"] == 250
+    hass.config_entries.async_update_entry(
+        entry, options={"room_entities": ["sensor.room", "sensor.new"]})
+    await hass.async_block_till_done()
+    assert hass.data[DOMAIN][entry.entry_id].learners["sensor.room"].count == 250