Validate projection fields from a machine-discoverable schema

[Widthdom]

Summary

Commands such as search, references, and map accept --fields bogus and succeed with rows like [{}]. At the same time, clients have no machine-readable catalog of valid projection fields.

Audit IDs: Q-02 and Q-18. Closed #1527 is adjacent schema/response work, but no exact issue covers this gap.

Expected behavior

Each command must validate requested fields against one authoritative registry and expose that registry to humans and automation.

PR scope

Keep this to the canonical projection-field registry, validation, and discovery. Row placement/JSON-mode serialization is a separate issue.

Implementation guide for Codex 5.6 sol xhigh

• Inventory fields per command/result kind and represent aliases/deprecations explicitly.
• Reuse the registry for validation, help, and either --fields list or a command-schema endpoint; do not maintain parallel lists.
• Unknown fields should return one stable usage error with valid/nearby candidates, never an empty object.
• Test command-specific fields, aliases, case behavior, multiple fields, JSON errors, and future extension.
• Update help/docs/completions as needed, run full tests, and add a bilingual changelog fragment.

Acceptance criteria

• Unknown fields fail deterministically with a typed error.
• Valid fields remain backward compatible.
• A machine client can discover the valid fields before executing the query.
• Help and runtime validation cannot drift independently.

[Widthdom]

Work started on branch fix-issue4836. I will implement the canonical projection-field registry, runtime validation, machine-readable discovery, tests, documentation, and a bilingual changelog fragment within this issue's scope.