Add support for continuous queries offset interval

Currently there is no way (with WindowNode) to specify an offset interval to alter the CQ’s default execution time.

```
stream
|window()
.period(1h)
.every(1h)
.align()
```

```
CREATE CONTINUOUS QUERY "cq_basic_offset" ON "transportation"
BEGIN
SELECT mean("passengers") INTO "average_passengers" FROM "bus_data" GROUP BY time(1h,15m)
END
```

Start by tracing the WindowNode handling and continuous-query execution path referenced in the issue, then compare the provided .align() and GROUP BY time(1h,15m) examples. Determine where an offset interval would be represented and applied, and add coverage showing that the continuous query executes at the requested offset.

Written by the indexing model from the issue text.