feat: pick definitions the selected fields do not reference

[barisakcam]

feat: pick definitions the selected fields do not reference

What

Adds @pick, a directive on the selection query that keeps definitions the selected fields do not reference — scalars, directives and enums that dependency-based filtering would otherwise drop.

query Selection
  @pick(
    enums: ["FuelType", "VehicleStatus"]
    scalars: ["DateTime"]
    directives: ["constraint"]
  ) {
  vehicle {
    id
    model
  }
}

Each argument takes three forms:

• Absent — only referenced definitions are kept, exactly as before.
• An empty list, such as scalars: [] — every definition of that kind is kept.
• A list of names — those definitions are kept in addition to the referenced ones.

An enum is always kept whole. Filtering never changes what an enum means, and @instanceTag(exclude: ...) remains the way to steer how an instance tag unfolds.

A query that only picks definitions has no fields to name, so an empty selection set is read as selecting nothing. That makes definition-only schemas filterable for the first time: on the QUDT units vocabulary, @pick(enums: [...], directives: []) {} reduces 712 enums to the three a model actually needs.

Why

A selection query could choose fields but had no way to retain an unreferenced definition, so filtered output silently lost scalars and directives that models still depended on.

Notes for review

• The directive is stripped from the document before validation rather than shipped as a schema definition, so nothing is added to the model and the name cannot collide with anything in it. The cost is that an external GraphQL tool reports an unknown directive for a selection query that uses one.
• A query without @pick produces byte-identical output to before.
• Enum value subsetting was implemented and then removed in fd3d829. It let a query change how an instance tag unfolds, which belongs to the model rather than to the query.
• All four places that read a selection query share one parser, including the one that reads a vendored dependency's query.

Testing

21 tests in tests/test_pick.py over a fixture covering picking, validation, the empty selection set, and equivalence with { __typename }. Full suite: 892 passing, mypy and pre-commit clean.