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>

README.md

@@ -74,7 +74,10 @@ is available only while it is `ready`. Useful attributes:
   `invalid_timestamp`, `future_timestamp` or `invalid_value_or_unit`),
   `age_seconds` (time since the sensor last reported to HA) and
   `max_age_seconds_24h` (largest age seen at polls in the last 24 hours; resets on
-  reload and can read up to one poll, 5 minutes, low).
+  reload and can read up to one poll, 5 minutes, low). Rooms also have `used`:
+  whether that room's reading was used in this poll.
+- `excluded_rooms` and `weight_coverage` — rooms left out of this poll, and the
+  share of the total comfort weight still covered by the rooms used (1.0 = all).
 - `heat_lag` and `heat_lag_uncertainty` — the radiator-lag estimate (0 cold, 1 fully
   warm) and its error bound. Learning waits while the bound is above 0.1, for
   example for about 30 minutes after an HA restart.
@@ -91,18 +94,31 @@ written to the Recorder; the other attributes are.
 ### Choosing freshness limits
 
 Room and weather report-age limits are set during setup and in **Configure**
-(5–1440 minutes; defaults 30 for rooms and 120 for weather). A room sensor older
-than its limit pauses learning and forecasts for **all** rooms until it reports
-again. A stale outdoor source falls back to the next one in its list; optional
-weather metrics never block. Recovery starts a new sampling interval, so nothing is
-learned across an outage.
+(5–1440 minutes; defaults 30 for rooms and 120 for weather).
 
-Within the limit, the last reading counts as current, so a too-generous limit lets
-a frozen value be learned for a while. A too-tight limit pauses the integration
-whenever a sensor is quietly between reports. Freshness uses HA's `last_reported`:
-the time a sensor last reported to HA, even with an unchanged value. It is not the
-age of the physical measurement, so a provider re-publishing cached data looks
-fresh, and sensors that report only on change can look stale in a steady room.
+A room sensor that is older than its limit, unavailable, missing or invalid is
+**left out**: the weighted temperature, learning and forecasts continue with the
+other rooms, whose weights are re-normalized. Status shows it in `excluded_rooms`.
+Learning and forecasts pause only when no room is usable, or when the relay,
+thermostat or outdoor temperature is missing. A room that reports again learns
+from its second poll onwards, so nothing is learned across its gap.
+
+The checkbox **Keep using the last value of a room sensor that stopped reporting**
+changes this for stale sensors: their last value is still used, for up to 24 hours
+without a report. Use it for sensors that report only when the value changes.
+Unavailable or invalid sensors are always left out, and so is any sensor silent
+for more than 24 hours, since it is most likely dead.
+
+A stale outdoor source falls back to the next one in its list; optional weather
+metrics never block.
+
+Within the limit (or with the checkbox on), the last reading counts as current,
+so a frozen value can be learned for a while. A too-tight limit (with the checkbox
+off) drops a room whenever its sensor is quietly between reports. Freshness uses
+HA's `last_reported`: the time a sensor last reported to HA, even with an unchanged
+value. It is not the age of the physical measurement, so a provider re-publishing
+cached data looks fresh, and sensors that report only on change can look stale in
+a steady room.
 
 After a day of running, set each limit comfortably above (about 1.5–2×) the largest
 `max_age_seconds_24h` among the sensors it covers. The relay and thermostat have no