Show learning activity with Learning Progress and Last Sample sensors

During a first test the Status entity only said "learning" for the
first half hour (the radiator-lag wait after a restart), and the
attributes explaining why were hard to find. Add two sensors:

- Learning Progress (%): for the least trained room, the mean of its
  three readiness requirements (200 samples, 30 ON, 30 OFF), each capped
  at 100 %; 100 % means ready. Attributes show the counts.
- Last Sample: an enum of the last poll's learning outcome, with English
  and Romanian state names (e.g. "Waiting for radiators to settle"). Its
  history shows when learning happens.

Polls blocked by missing shared inputs now report waiting_for_inputs.
Readiness thresholds move to const.py.

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

custom_components/heating_mpc/strings.json

@@ -104,5 +104,21 @@
       "invalid_rooms": "Select 1–12 distinct room sensors.",
       "invalid_weather": "Select at least one outdoor temperature sensor, distinct from room sensors."
     }
+  },
+  "entity": {
+    "sensor": {
+      "last_sample": {
+        "state": {
+          "waiting_for_inputs": "Waiting for inputs",
+          "first_sample": "First sample",
+          "learned": "Learned",
+          "partly_rejected": "Partly rejected (outlier)",
+          "interval_out_of_range": "Irregular interval",
+          "relay_changed": "Relay switched",
+          "outside_source_changed": "Outdoor source changed",
+          "lag_uncertain": "Waiting for radiators to settle"
+        }
+      }
+    }
   }
 }