Aggregate textification renders expression indices as field references

[wackywendell]

Summary

Aggregate textification incorrectly renders grouping expression references as field references when grouping expressions are non-trivial (e.g., function calls). This causes roundtrip failures.

Problem

The Substrait proto AggregateRel uses a level of indirection for groupings:

• grouping_expressions: list of expressions (e.g., [abs($0)])
• groupings[].expression_references: indices into grouping_expressions (e.g., [0])

The textifier builds arguments from expression_references as Value::Reference(i), which renders as $i. But $i in our text format means "field i of the input relation", not "grouping expression i". When grouping expressions are bare field refs like $0, the output is coincidentally correct. But for complex expressions like abs($0), the textifier produces $0 instead of abs($0).

Expected output:

Aggregate[abs($0) => abs($0), count($1)]

Actual output:

Aggregate[$0 => abs($0), count($1)]

Failing test

A failing roundtrip test is on branch wendell/aggregate-expr-grouping-bug (test_aggregate_expression_grouping_roundtrip in tests/plan_roundtrip.rs).

Relevant proto fields: AggregateRel.grouping_expressions and Grouping.expression_references.

Goal

The textifier should resolve expression_references indices through grouping_expressions to render the actual expressions in the arguments section, rather than emitting the raw index as a field reference.

Success Criteria

• test_aggregate_expression_grouping_roundtrip passes
• Existing aggregate tests continue to pass