perf(concurrency): use safe auto worker limits by default

[luojiyin1987]

Summary

• Make an omitted --threads option use auto.
• Cap auto workers at 4 for files below 1 MiB.
• Keep the existing caps of 2 and 1 for larger files.
• Let explicit numeric values override all automatic caps.
• Show small-file limits in development diagnostics.
• Document the default worker limits in the README.

Benchmark basis

The memory benchmark matrix supports a four-worker small-file cap.

At 900 KiB, four workers used about 2.31 GiB median peak RSS.
Eight workers used about 3.92 GiB median peak RSS.

At 1023 KiB, four workers used about 2.82 GiB median peak RSS.
Eight workers used about 4.03 GiB median peak RSS.

The default policy stays conservative.
Users can pass --threads N when they accept the resource cost.

Validation

• npm run lint
• npm test -- --runInBand
• node ./lib/src/lint-md.js README.md
• 26 test suites passed.
• 223 tests passed.

Redactions

None.