Join Node should support fill `previous`.

This should be possible.
```
active
|join(unique)
.as('active', 'unique')
.fill('previous')
```

The issue provides a Kapacitor stream example using a Join Node, unique fields, and fill('previous'), but names no files, tests, or entry points. Start by locating Join Node handling and existing fill behavior, then determine the expected result for previous-value filling and add coverage for the shown pipeline.

Written by the indexing model from the issue text.