Keep bounded-response preflight failures machine-readable and actionable

[Widthdom]

Summary

Map and recipes byte-budget preflight failures print human text under --json; map also says the budget is too small without reporting the minimum usable byte floor.

Audit IDs: R-12. Reproduced with locally built cdidx v1.40.3 at 404db09489c305f17d94f37026e00564d3ac527a.

Existing issue relationship

Follow-up adjacent to closed #4564 and #4319.

Expected behavior and PR scope

One shared budget-preflight envelope should include requested/effective/minimum bytes, stable error code/category, and retry guidance. Keep this to preflight errors across bounded responders.

Implementation guide for Codex 5.6 sol xhigh

• Compute/return the minimum required complete envelope/row where possible, with uncertainty explicitly represented.
• Use the normal machine error writer and keep stdout/stderr contracts parseable.
• Cover zero/tiny/exact-minimum budgets, empty/non-empty, Unicode/escaping, map/recipes and representative shared users.
• Never attempt to emit a normal payload that cannot fit after preflight.
• Add focused positive/negative regression tests, run the full net8.0 and net9.0 suite, and add the required bilingual changelog.d/unreleased/ fragment.

Acceptance criteria

• JSON mode always emits valid structured errors.
• Errors state a usable minimum or why it cannot be known.
• Retry hints are command-specific and machine-readable.