test: guard against sub-unit batch sizes

[vycdev2]

Summary

• Add a regression test for sub-unit numeric and string batch sizes. Both must use the positive default rather than floor to zero, which would prevent file-scan loops from advancing.
• No production behavior changes or changelog entry.

Verification

• npm run typecheck — passed.
• npm test — passed (416 tests).
• npm run compile — passed.
• npm run format:check — passed.
• git diff --check — passed.

Risk

• Low: test-only change.

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.