[VL] Eager aggregation: Push group-only aggregation

[zhztheplayer]

This PR pushes grouping-only aggregation below both sides of an inner equi-join to reduce duplicate rows entering the join.

For example:

SELECT DISTINCT l.name, r.category
FROM l JOIN r ON l.id = r.id;

Before:
Distinct [name, category]
  Join [l.id = r.id]
    l
    r

After:
Distinct [name, category]
  Join [l.id = r.id]
    Distinct [id, name]
      l
    Distinct [id, category]
      r

The lower aggregates retain the join keys, and the top DISTINCT preserves the original result.

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