Startup stagger logic between config entries never fires #8

Bug

In `init.py` (~line 179), the anti-hammer stagger delay for multiple config entries sharing one Modbus link checks:

```python
existing_same_link = sum(1 for c in hass.data[DOMAIN][CONTROLLER].values() if getattr(c, "connection_id", None) == connection_id)
```

But the objects stored in `hass.data[DOMAIN][CONTROLLER]` are `SolisInverterHub` instances, which have no `connection_id` attribute (only the concrete `ModbusController`/`SolisCloudController` do). `getattr(..., None)` always returns `None`, so `existing_same_link` is always `0` and the stagger delay never applies.

Impact

Two inverters sharing the same Modbus host:port (or serial port) can start polling simultaneously with no stagger, which the code clearly intended to prevent.

Fix

Either walk into `hub.modbus.connection_id` / `hub.cloud` as appropriate, or expose a `connection_id` passthrough property on `SolisInverterHub`.

Found during a full repo review — see `WORKING_NOTES.md`, "Repo Review 2026-07-03", bug #4.