Outer Join post-5.0 Improvements

Citus 5.0 has partial support for outer join queries. We'd now like to refactor and expand on this feature's scope.
- Design unified join order planner (high priority)
- Implement unified join order planner (high priority)
- Generate replacement for prunable outer join (e.g. join with empty table)
- Do not push down filters on outer join output in re-partition joins
- Create bucket(s) for non-matching values in single-repartition outer joins
- Integration with subqueries

Start by reading the existing Citus 5.0 outer-join support and the join-order planner entry points. Trace each listed area—unified planning, pruning, filter pushdown, non-matching buckets, and subqueries—to determine the affected components. Done requires a design and implementation covering the full scope, with validation for each outer-join case.

Written by the indexing model from the issue text.