Add bounded JSON output to unused analysis

[Widthdom]

Summary

unused has cursor/summary modes but no --max-json-bytes; --by-bucket can emit thousands of rows, and the current Markdown suppression population alone is 1,683.

Audit IDs: R-07. Reproduced with locally built cdidx v1.40.3 at 404db09489c305f17d94f37026e00564d3ac527a.

Existing issue relationship

Residual/follow-up to closed #4344; closed #4585 is adjacent bounded-output work.

Expected behavior and PR scope

Apply the common whole-row byte budget to normal and by-bucket unused JSON while preserving continuation/completion metadata. Response-shape duplication is tracked separately.

Implementation guide for Codex 5.6 sol xhigh

• Measure final UTF-8 bytes and emit only complete rows/envelopes.
• Bind cursor to filters/buckets/order/index generation and expose returned/omitted/has_more.
• Return a typed minimum-budget error when required envelope/first row cannot fit.
• Cover compact/full/by-bucket, huge signatures, Unicode, exact boundaries, final/empty pages, and stale cursors.
• 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

• Unused JSON respects the requested budget.
• Every partial response remains valid and resumable.
• Unbounded behavior and selection semantics remain unchanged.

[Widthdom]

Work started on branch fix-issue4905.