Home Assistant 2026.9 deprecates the `via_device` (domain, identifier)
tuple in favour of `via_device_id`, which takes the parent device's
registry id; `via_device` stops working in 2027.8.0.
Register the hub device explicitly in async_setup_entry before the
platforms are forwarded, so the per-identity entities can look it up and
pass its `.id`. The hub device info moves into a shared helper so both
call sites describe the same device.