HA entity, sensor and device classes (iteration 2, step 3)

HaEntity describes an entity for device-based discovery, throttles its
state (SEN-14) and reports availability (SEN-15); HaSensor is the
`sensor` type. HaDevice publishes discovery, availability and states
after each connect and after HA's birth message, provides the last
will and removes itself from HA. Unit tests with a fake MQTT client.

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

docs/PROGRESS.md

@@ -23,8 +23,9 @@ CLAUDE.md ("Working rules").
   The error itself is covered off the device by `tst_config`. Checks 3 and 4 run in the next
   session on the Frame (iteration 2 testing needs it too).
 - **Implementation** follows the order in DESIGN.md (steps 1–7). Steps 1 (libmosquitto in
-  CMake and the containers) and 2 (`MqttClient`, `MosquittoClient`, `tst_mqtt`) done; next:
-  step 3 (`HaEntity`, `HaSensor`, `HaDevice`, unit tests).
+  CMake and the containers), 2 (`MqttClient`, `MosquittoClient`, `tst_mqtt`) and 3
+  (`HaEntity`, `HaSensor`, `HaDevice`, unit tests) done; next: step 4 (app wiring,
+  `--desktop-novr`, UI-1).
 
 ## Next steps
 
@@ -144,6 +145,10 @@ Known limitation: the bundled-Qt package does not support WebEngine in this iter
   host 3 × 12/12; `container-test.sh` 9/9 (Debian's libmosquitto 2.0.21), no Vulkan
   validation messages; cross build: private Qt symbols only `QVulkan*`, at most
   `GLIBC_2.34`; the app binary links libmosquitto only from step 4 on (first use).
+- Step 3: `HaEntity` (description, discovery component, throttling, availability),
+  `HaSensor`, `HaDevice` (discovery payload, publish all after connect and after HA's birth
+  message with random delay, last will, `removeFromHa()`), `FakeMqttClient` for tests.
+  `tst_haentity` 11/11, `tst_hadevice` 14/14 on the host.
 
 ### 2026-10-05: requirements approved