feat: support explicit emits for passthrough relations

[wackywendell]

This PR adds compact and explicit output syntax for pass-through relations:

Filter, Sort, Fetch, Set, and Cross.

For example, a Filter with an explicit output mapping can be written as:

Filter[gt($0, 0):boolean |> $1, $0]

Compact formatting remains the default. OutputOptions::show_emit and OutputOptions::verbose() select explicit formatting, preserving an explicit identity Emit mapping rather than normalizing it to Direct.

Part of #34.

Changes

• Add => compact output and |> explicit output parsing for pass-through relations, including empty mappings written as _.
• Format pass-through relations without an output clause for Direct, with compact => output by default and explicit |> output when requested.
• Add OutputOptions::show_emit as the public formatting option for explicit relation output.
• Model output formatting around the text syntax: OutputAdditions for +>, OutputMapping for |>, Emitted for compact final output, and OutputClause for their composition.
• Document Substrait's direct-output and emit model, including input-order and direct-output-order field-reference scopes.
• Accept and canonically format empty named-Read additions as Read[table +> _]; retain the previous blank spelling as accepted input for now.

API Impact

• OutputOptions now exposes show_emit: bool.
• OutputOptions::verbose() sets show_emit to true.

Validation

• just test
• just check

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Validate explicit emit mappings before writing references

When show_emit/OutputOptions::verbose() is used on a Filter/Sort/Fetch/Set/Cross protobuf whose RelCommon.Emit contains an out-of-range or negative index, this explicit path writes $N directly and records no FormatError; the compact Emitted path still checks the index against the direct output, so the same malformed input can silently produce invalid text only in the new explicit output mode. Please make this path perform the same bounds check while still best-effort formatting.

AGENTS.md reference: AGENTS.md:L57-L58

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