Add aliasing support to REPL select() and group_by()

[aisrael]

Summary

• select() arguments (plain columns or aggregates) can be renamed via name: value keyword syntax, e.g. select(:foo, foo_bar: :bar, total: sum(:qty)). Quoted names ("name with space": value) are supported for names that aren't valid bare identifiers.
• Works for plain projections, global aggregates, grouped aggregates, and ORC's plain-column select path.
• group_by() keys can carry their own alias (e.g. group_by(key: :foo)), which becomes the default output name for that key. select() may refer to the key by its underlying column or by the group_by() alias; a matching select() alias takes precedence when present.

Changes

• src/pipeline/spec.rs: parse/model aliases for select() args and group_by() keys
• src/pipeline/record_batch.rs, src/pipeline/dataframe/transform.rs: apply aliases when building output batches/dataframes
• src/pipeline/builder.rs, src/pipeline.rs: wire alias info through the pipeline builder
• src/cli/repl/plan.rs, src/cli/repl/stage.rs, src/cli/repl/builder_bridge.rs, src/cli/repl/mod.rs: REPL-side plumbing for alias syntax
• features/repl/select.feature: Cucumber scenarios covering aliasing behavior
• docs/REPL.md, CHANGELOG.md: document the new syntax and behavior

Testing

• cargo check --all-targets passes
• Run cargo test and the Cucumber suite (features/repl/select.feature) to verify aliasing behavior end to end

Checklist

• Tests pass
• Documentation updated
• No breaking changes