Reject invisible graph snippet lengths

[Widthdom]

Summary

• Reject explicit --snippet-lines for references, callers, and callees unless --body and body-capable text or JSON output are selected.
• Preserve zero/range errors, verbatim option-like queries, bounded total counts, and cursor continuation while failing invalid combinations before database access.
• Add shared command/format coverage, bounded replay regressions, bilingual documentation/help updates, and a changelog fragment.

Validation

• Release focused regression set on the final branch: 21 passed on net8.0 and 21 passed on net9.0.
• Full net8.0 suite during implementation: 10,836 passed, 7 skipped, 1 known watcher failure (Stabilize net8 subproject watcher reconciliation for ancestor .gitignore changes #4955).
• Full net9.0 suite during implementation: 10,362 passed, 420 skipped, 1 known watcher failure (Investigate net9 ancestor .gitignore watcher reconciliation timeout #4966).
• dotnet build CodeIndex.sln -c Release --no-restore
• dotnet build CodeIndex.sln -c Debug --no-restore
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• dotnet run --project tools/CodeIndex.Changelog -- check
• Local index freshness/readiness: workspace matched, no failed checks, fold and graph ready.
• Codex adversarial review: 2 rounds completed; all 4 actionable findings were fixed and regression-tested.

Documentation and changelog

• Updated matching English/Japanese sections in DEVELOPER_GUIDE.md and TESTING_GUIDE.md.
• Added changelog.d/unreleased/4882.fixed.md.

Follow-ups

• Stabilize net8 subproject watcher reconciliation for ancestor .gitignore changes #4955
• Investigate net9 ancestor .gitignore watcher reconciliation timeout #4966

Fixes #4882