Add integration icon

Home Assistant 2026.3+ shows custom integrations' own brand images from
custom_components/<domain>/brand/. Add a radiator with a forecast curve
(solid past, "now" marker, dashed prediction) as icon.png/icon@2x.png
(256/512 px, transparent) plus dark-theme variants, so the integration
is easy to spot in Devices & services. Older HA versions ignore the
folder and show the generic placeholder; the integration is unaffected.

scripts/make_brand_icons.py regenerates the images with Pillow from the
test requirements; the package check verifies sizes and transparency.
Listed under 0.2.3, which has not been deployed yet.

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

README.md

@@ -14,6 +14,8 @@ This project targets Home Assistant OS on a Raspberry Pi 5, with one existing ON
 6. Choose *ordered lists* of **outdoor sensor entity IDs**. First valid sensor is active; if missing, unavailable, nonnumeric or outside reasonable bounds, the next valid sensor is used automatically.
 7. Assign a relative comfort weight (0.1–10) to each selected room.
 
+On Home Assistant 2026.3 or newer the integration shows its own icon (a radiator with a forecast curve) in **Devices & services**; older versions show a generic placeholder.
+
 **No entity IDs are hard-coded.** Every room and outdoor measurement can be changed later via **Configure**, without editing YAML or Python.
 
 ## Reconfigurable weather inputs
@@ -155,6 +157,10 @@ run without Home Assistant installed:
 python -m unittest discover -s tests -v
 ```
 
+The brand icons in `custom_components/heating_mpc/brand/` are drawn by
+`python scripts/make_brand_icons.py` (uses Pillow from the test requirements);
+the package check verifies their sizes and transparency.
+
 HA tests cover setup, config/options forms, migration, reload and entity cleanup,
 scheduled sampling and poll cancellation, the relay listener, stored model
 restoration, retention and deletion, missing inputs, manual off/presets, optimizer