Emit one stable error for invalid outline projections

[Widthdom]

Summary

--outline-fields bogus emits two top-level failures: “unsupported field” and then “requires at least one field.” Secondary validation runs after the primary field error.

Audit IDs: Q-11. Reproduced with locally built cdidx v1.40.3 at 404db09489c305f17d94f37026e00564d3ac527a.

Existing issue relationship

No exact existing issue was found.

Expected behavior and PR scope

Invalid projection input should fail once with one stable code/category, valid candidates, and usage/hint. Keep this to outline projection validation order.

Implementation guide for Codex 5.6 sol xhigh

• Normalize and validate field names before checking whether the resulting selection is empty.
• Represent validation failures structurally and stop downstream validators after a terminal error.
• Cover one/multiple unknown fields, empty lists, aliases, duplicates, JSON/human output, and mixed valid/invalid input.
• Add focused regression/contract tests, run the full net8.0 and net9.0 suite, and add the required bilingual changelog.d/unreleased/ fragment.

Acceptance criteria

• The reproduction emits exactly one top-level error.
• The error names the bad field and valid alternatives.
• Valid and deliberately empty projections keep documented behavior.

[Widthdom]

Work started on branch fix-issue4876. I will keep the implementation scoped to outline projection validation order and add focused tests plus the bilingual changelog fragment.