Improve thread safety and formatter performance

[nikhilvdev]

Lock shared transport buffers and file writes for concurrent logging, add an optional orjson JSON backend, and cache bounded OptLogger views. Update documentation, benchmarks, and CI for free-threaded Python; switch new releases to FSL-1.1-MIT.

Summary

Checklist

• ruff check . passes
• mypy logquill passes (--strict on the public API)
• pytest passes, and new surface area has test coverage
• Public API additions are documented in README.md with a runnable example
• CHANGELOG.md has an entry under Unreleased
• If this touches the cross-language contract (levels, logger methods,
  record shape, transport/plugin contracts, config shape) shared with
  logquill-js, a matching tracking issue/PR is opened there

Scope