Leave out unusable rooms instead of pausing; add keep-last-value option

A single stale, unavailable, missing or invalid room sensor paused
learning and forecasts for every room. That is not acceptable once the
MPC may control heating.

Now such a room is left out: the weighted temperature, learning and the
optimizer use the remaining rooms with their weights re-normalized, and
a room learns only across poll pairs where it was usable at both ends.
Readiness considers only the rooms used. Everything pauses only when no
room is usable or a shared input (relay, thermostat, outdoor
temperature) is missing.

A new setup/Configure checkbox, room_stale_use_last_value (default off),
keeps using a stale room sensor's last value instead, for sensors that
report only on change. It never applies to unavailable or invalid
sensors, or to a sensor silent for more than 24 hours.

Status adds excluded_rooms, weight_coverage and a per-room "used" flag.
English and Romanian labels and help text added.

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

custom_components/heating_mpc/translations/en.json

@@ -10,7 +10,11 @@
           "heater_entity": "Physical heating switch (read-only)",
           "room_entities": "Individual room temperature sensors",
           "room_max_age_minutes": "Maximum room sensor report age (minutes)",
-          "weather_max_age_minutes": "Maximum weather sensor report age (minutes)"
+          "weather_max_age_minutes": "Maximum weather sensor report age (minutes)",
+          "room_stale_use_last_value": "Keep using the last value of a room sensor that stopped reporting"
+        },
+        "data_description": {
+          "room_stale_use_last_value": "Off: a room sensor older than its report-age limit is left out and the other rooms are re-weighted. On: its last value is still used, for up to 24 hours without a report; for sensors that report only when the value changes. Unavailable or invalid room sensors are always left out. Nothing pauses unless no room is usable."
         }
       },
       "weather": {
@@ -63,7 +67,11 @@
           "heating_cost": "Heating runtime cost",
           "comfort_band": "Comfort tolerance (°C)",
           "room_max_age_minutes": "Maximum room sensor report age (minutes)",
-          "weather_max_age_minutes": "Maximum weather sensor report age (minutes)"
+          "weather_max_age_minutes": "Maximum weather sensor report age (minutes)",
+          "room_stale_use_last_value": "Keep using the last value of a room sensor that stopped reporting"
+        },
+        "data_description": {
+          "room_stale_use_last_value": "Off: a room sensor older than its report-age limit is left out and the other rooms are re-weighted. On: its last value is still used, for up to 24 hours without a report; for sensors that report only when the value changes. Unavailable or invalid room sensors are always left out. Nothing pauses unless no room is usable."
         }
       },
       "weather": {