The left-core migration decided "this home used Home Assistant" with its own config reader: the
first platforms.homeassistant block in config.yaml. The pre-extraction gateway enabled the adapter
from more places: the legacy gateway.json base layer, the gateway.homeassistant shorthand, and
nested gateway.platforms + top-level platforms blocks merged key by key. Those homes got no plugin
and no notice, and lost Home Assistant on update.
platform_configured_on() now asks the gateway loader itself: load_legacy_gateway_json +
read_yaml_layers (managed overlay, ${VAR} expansion) + merge_platform_sections, then
PlatformConfig.from_dict for the enabled flag, so the last explicit `enabled` keeps winning
(config.yaml enabled: false over a gateway.json enable stays not-in-use).
merge_platform_sections takes `also=` so the gateway.<platform> shorthand of a platform that
left core counts before its plugin registers (Platform() no longer knows the name).
2da37476a5