Add bounded JSON output to unused

[Widthdom]

Summary

• add --max-json-bytes support to unused through the shared bounded-response envelope
• preserve whole UTF-8 rows, continuation/completion metadata, and generation-bound response:v2 cursors for full, compact, and --by-bucket output
• recompute returned bucket, confidence, and contract-domain summaries after byte trimming
• add focused Unicode, long-signature, exact-boundary, minimum-budget, final-page, and stale/mismatched-cursor coverage
• document the contract in both English and Japanese and add changelog.d/unreleased/4905.added.md

Why

unused JSON previously had no whole-response byte budget, so large audits—especially --by-bucket output—could exceed automation and agent context limits. The command did not opt into the shared bounded-response path, and its query layer did not consume a shared response offset.

Impact

Callers can now request a hard JSON byte cap that includes the final newline. Partial responses contain only complete rows and can be resumed without gaps or duplicates. Budgets too small for metadata plus one row return a typed usage error with empty stdout. Uncapped output remains compatible.

Validation

• dotnet test CodeIndex.sln -c Release -f net8.0 --no-restore -p:UseSharedCompilation=false --logger 'console;verbosity=minimal' — 11,012 passed, 7 skipped
• dotnet test CodeIndex.sln -c Release -f net9.0 --no-restore -p:UseSharedCompilation=false --logger 'console;verbosity=minimal' — 10,538 passed, 420 skipped
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --no-restore --filter 'FullyQualifiedName~Issue4905' — 5 passed on net8.0 and 5 passed on net9.0
• broader QueryCommandRunnerUnused / JsonEnvelopeWrapper regression selection — 59 passed on net8.0
• dotnet format CodeIndex.sln --no-restore --verify-no-changes
• dotnet run --project tools/CodeIndex.Changelog -- check
• cdidx status --check --json
• mandatory codex review --base origin/main — no blocking or actionable issues found

Scope

Response-shape duplication remains separately tracked in #4904.

Fixes #4905